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MySQL），是以类似表格的扁平化方式存储数据。因此不会直接将树结构存储在数据库中，通常是通过邻接表、路径枚举、嵌套集或闭包表来存储。</p><p>其中，邻接表是最常用的方案之一，其存储模型如下：</p><table><thead><tr><th>id</th><th>pid</th><th>data</th></tr></thead><tbody><tr><td>1</td><td>0</td><td>a</td></tr><tr><td>2</td><td>1</td><td>b</td></tr><tr><td>3</td><td>1</td><td>c</td></tr></tbody></table><p>该模型代表了如下的树状结构：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token literal-property property" style="color:#36acaa">id</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">1</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token literal-property property" style="color:#36acaa">pid</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">0</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token literal-property property" style="color:#36acaa">data</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&#x27;a&#x27;</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token literal-property property" style="color:#36acaa">children</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token punctuation" style="color:#393A34">{</span><span class="token literal-property property" style="color:#36acaa">id</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">2</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">pid</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">1</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">data</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&#x27;b&#x27;</span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token punctuation" style="color:#393A34">{</span><span class="token literal-property property" style="color:#36acaa">id</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">3</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">pid</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">1</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">data</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&#x27;c&#x27;</span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>大部分情况下，会交给应用程序来构造树结构。</p><h2 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="典型题目">典型题目<a href="#典型题目" class="hash-link" aria-label="Direct link to 典型题目" title="Direct link to 典型题目"></a></h2><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> list </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">pid</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">id</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">1</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">data</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;1&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">pid</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">1</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">id</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">2</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">data</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;2-1&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">pid</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">1</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">id</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">3</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">data</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;2-2&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">pid</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">2</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">id</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">4</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">data</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;3-1&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">pid</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">3</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">id</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">5</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">data</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;3-2&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">pid</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">4</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">id</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">6</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">data</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;4-1&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><h2 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="解法">解法<a href="#解法" class="hash-link" aria-label="Direct link to 解法" title="Direct link to 解法"></a></h2><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="解法一">解法一<a href="#解法一" class="hash-link" aria-label="Direct link to 解法一" title="Direct link to 解法一"></a></h3><p>递归解法：该方法简单易懂，从根节点出发，每一轮迭代找到 pid 为当前节点 id 的节点，作为当前节点的 children，递归进行。</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">listToTree</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  list</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  pid </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> idName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;id&quot;</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> pidName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;pid&quot;</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> childName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;children&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> list</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">reduce</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">root</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> item</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=&gt;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token comment" style="color:#999988;font-style:italic">// 遍历每一项，如果该项与当前 pid 匹配，则递归构建该项的子树</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">item</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">pidName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">===</span><span class="token plain"> pid</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> children </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">listToTree</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">list</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> item</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">idName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">children</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">length</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">        item</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">childName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> children</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token spread operator" style="color:#393A34">...</span><span class="token plain">root</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> item</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> root</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>时间复杂度分析：最坏的情况下，这棵树退化为链表，且倒序排列。每一轮迭代需要在最后面才找到目标节点。假设有 n 个元素，那么总迭代次数为 <code>n+(n-1) + (n-2) + ... + 1</code>，时间复杂度为 O(n^2)。</p><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="解法二">解法二<a href="#解法二" class="hash-link" aria-label="Direct link to 解法二" title="Direct link to 解法二"></a></h3><p>迭代法：利用对象在 js 中是引用类型的原理。第一轮遍历将所有的项，将项的 id 与项自身在字典中建立映射，为后面的立即访问做好准备。
由于操作的每一项都是对象，结果集 root 中的每一项和字典中相同 id 对应的项实际上指向的是同一块数据。后续的遍历中，直接对字典进行操作，操作同时会反应到 root 中。</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">listToTree</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  list</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  rootId </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> idName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;id&quot;</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> pidName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;pid&quot;</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> childName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;children&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> record </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 用空间换时间，用于将所有项的 id 及自身记录到字典中</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> root </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  list</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">forEach</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">item</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=&gt;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">item</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">idName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> item</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 记录 id 与项的映射</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    item</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">childName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  list</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">forEach</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">item</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=&gt;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">item</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">pidName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">===</span><span class="token plain"> rootId</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      root</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">push</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">item</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">else</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token comment" style="color:#999988;font-style:italic">// 由于持有的是引用，record 中相关元素的修改，会在反映在 root 中。</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">item</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">pidName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">childName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">push</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">item</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> root</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>record 字典 与 root 结果集的参考内存引用关系如图所示：</p><p><img loading="lazy" src="https://user-images.githubusercontent.com/4338052/129750250-f447f56b-4342-4130-b82f-af017bd7c3c7.png" alt="image" class="img_ev3q"></p><p>时间复杂度分析：经历了两轮迭代，假设有 n 个元素，那么总迭代次数为 <code>n + n</code>，时间复杂度为 O(n)。</p><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="解法二变体">解法二变体<a href="#解法二变体" class="hash-link" aria-label="Direct link to 解法二变体" title="Direct link to 解法二变体"></a></h3><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="变体一">变体一<a href="#变体一" class="hash-link" aria-label="Direct link to 变体一" title="Direct link to 变体一"></a></h4><p>在解法二的基础上，将两轮迭代合并成一轮迭代。采用边迭代边构建的方式：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">listToTree</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  list</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  rootId </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> idName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;id&quot;</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> pidName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;pid&quot;</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> childName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;children&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> record </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 用空间换时间，用于将所有项的 id 及自身记录到字典中</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> root </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  list</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">forEach</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">item</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=&gt;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> id </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> item</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">idName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> parentId </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> item</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">pidName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token comment" style="color:#999988;font-style:italic">// 如果该项不在 record 中，则放入 record。如果该项已存在 （可能由别的项构建 pid 加入），则合并该项和已存在的数据</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">id</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">!</span><span class="token plain">record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">id</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">?</span><span class="token plain"> item </span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token spread operator" style="color:#393A34">...</span><span class="token plain">item</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token spread operator" style="color:#393A34">...</span><span class="token plain">record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">id</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> treeItem </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">id</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">parentId </span><span class="token operator" style="color:#393A34">===</span><span class="token plain"> rootId</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token comment" style="color:#999988;font-style:italic">// 如果是根元素，则加入结果集</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      root</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">push</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">treeItem</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">else</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token comment" style="color:#999988;font-style:italic">// 如果父元素不存在，则初始化父元素</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token operator" style="color:#393A34">!</span><span class="token plain">record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">parentId</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">        record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">parentId</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token comment" style="color:#999988;font-style:italic">// 如果父元素没有 children, 则初始化</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token operator" style="color:#393A34">!</span><span class="token plain">record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">parentId</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">childName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">        record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">parentId</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">childName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">parentId</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">childName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">push</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">treeItem</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> root</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>时间复杂度分析：经历了一轮迭代，假设有 n 个元素，那么时间复杂度为 O(n)。</p><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="变体二">变体二<a href="#变体二" class="hash-link" aria-label="Direct link to 变体二" title="Direct link to 变体二"></a></h4><p>record 字典仅记录 id 与 children 的映射关系，代码更精简：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">listToTree</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  list</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  rootId </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> idName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;id&quot;</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> pidName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;pid&quot;</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> childName </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;children&quot;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> record </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 用空间换时间，仅用于记录 children</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> root </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  list</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">forEach</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">item</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=&gt;</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> newItem </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token known-class-name class-name">Object</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">assign</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">{</span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> item</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 如有需要，可以复制 item ，可以不影响 list 中原有的元素。</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> id </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> newItem</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">idName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> parentId </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> newItem</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">pidName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token comment" style="color:#999988;font-style:italic">// 如果当前 id 的 children 已存在，则加入 children 字段中，否则，初始化 children</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token comment" style="color:#999988;font-style:italic">// item 与 record[id] 引用同一份 children，后续迭代中更新 record[parendId] 就会反映到 item 中</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    newItem</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">childName</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">id</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">?</span><span class="token plain"> record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">id</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">id</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">parentId </span><span class="token operator" style="color:#393A34">===</span><span class="token plain"> rootId</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      root</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">push</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">newItem</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">else</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token operator" style="color:#393A34">!</span><span class="token plain">record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">parentId</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">        record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">parentId</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">      record</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">parentId</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">push</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">newItem</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">    </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">  </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> root</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>时间复杂度分析：经历了一轮迭代，假设有 n 个元素，那么时间复杂度为 O(n)。</p><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="代码演示及总结">代码演示及总结<a href="#代码演示及总结" class="hash-link" aria-label="Direct link to 代码演示及总结" title="Direct link to 代码演示及总结"></a></h3><p><a href="https://codesandbox.io/s/practical-sun-vo4yi?file=/src/App.js" target="_blank" rel="noopener noreferrer">Code Sandbox - List to Tree</a></p><ul><li>递归法：在数据量增大的时候，性能会急剧下降。好处是可以在构建树的过程中，给节点添加层级信息。</li><li>迭代法：速度快。但如果想要不影响源数据，需要在 record 中存储一份复制的数据，且无法在构建的过程中得知节点的层级信息，需要构建完后再次深度优先遍历获取。</li><li>迭代法变体一：按需创建 children，可以避免空的 children 列表。</li></ul></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages navigation"><a class="pagination-nav__link pagination-nav__link--prev" href="/awesome-interview/book3/network-http-1-2"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">网络：HTTP2 和 HTTP1.1 的对比</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/awesome-interview/book3/algorithm-binary-tree-k"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">算法：二叉搜索树的第 k 个结点</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents 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