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Lowest Common Ancestor of a Binary Tree

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Description

tab: English
 
<p>Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.</p>
 
<p>According to the <a href="https://en.wikipedia.org/wiki/Lowest_common_ancestor" target="_blank">definition of LCA on Wikipedia</a>: &ldquo;The lowest common ancestor is defined between two nodes <code>p</code> and <code>q</code> as the lowest node in <code>T</code> that has both <code>p</code> and <code>q</code> as descendants (where we allow <b>a node to be a descendant of itself</b>).&rdquo;</p>
 
<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/14/binarytree.png" style="width: 200px; height: 190px;" />
<pre>
<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
<strong>Output:</strong> 3
<strong>Explanation:</strong> The LCA of nodes 5 and 1 is 3.
</pre>
 
<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/14/binarytree.png" style="width: 200px; height: 190px;" />
<pre>
<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
<strong>Output:</strong> 5
<strong>Explanation:</strong> The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.
</pre>
 
<p><strong class="example">Example 3:</strong></p>
 
<pre>
<strong>Input:</strong> root = [1,2], p = 1, q = 2
<strong>Output:</strong> 1
</pre>
 
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>
 
<ul>
	<li>The number of nodes in the tree is in the range <code>[2, 10<sup>5</sup>]</code>.</li>
	<li><code>-10<sup>9</sup> &lt;= Node.val &lt;= 10<sup>9</sup></code></li>
	<li>All <code>Node.val</code> are <strong>unique</strong>.</li>
	<li><code>p != q</code></li>
	<li><code>p</code> and <code>q</code> will exist in the tree.</li>
</ul>
 
 
 
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[submissions](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/submissions/) | [solutions](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/solutions/)
 
 
tab: 中文
 
<p>给定一个二叉树, 找到该树中两个指定节点的最近公共祖先。</p>
 
<p><a href="https://baike.baidu.com/item/%E6%9C%80%E8%BF%91%E5%85%AC%E5%85%B1%E7%A5%96%E5%85%88/8918834?fr=aladdin" target="_blank">百度百科</a>中最近公共祖先的定义为：“对于有根树 T 的两个节点 p、q，最近公共祖先表示为一个节点 x，满足 x 是 p、q 的祖先且 x 的深度尽可能大（<strong>一个节点也可以是它自己的祖先</strong>）。”</p>
 
<p> </p>
 
<p><strong>示例 1：</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/14/binarytree.png" style="width: 200px; height: 190px;" />
<pre>
<strong>输入：</strong>root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
<strong>输出：</strong>3
<strong>解释：</strong>节点 <code>5 </code>和节点 <code>1 </code>的最近公共祖先是节点 <code>3 。</code>
</pre>
 
<p><strong>示例 2：</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/14/binarytree.png" style="width: 200px; height: 190px;" />
<pre>
<strong>输入：</strong>root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
<strong>输出：</strong>5
<strong>解释：</strong>节点 <code>5 </code>和节点 <code>4 </code>的最近公共祖先是节点 <code>5 。</code>因为根据定义最近公共祖先节点可以为节点本身。
</pre>
 
<p><strong>示例 3：</strong></p>
 
<pre>
<strong>输入：</strong>root = [1,2], p = 1, q = 2
<strong>输出：</strong>1
</pre>
 
<p> </p>
 
<p><strong>提示：</strong></p>
 
<ul>
	<li>树中节点数目在范围 <code>[2, 10<sup>5</sup>]</code> 内。</li>
	<li><code>-10<sup>9</sup> <= Node.val <= 10<sup>9</sup></code></li>
	<li>所有 <code>Node.val</code> <code>互不相同</code> 。</li>
	<li><code>p != q</code></li>
	<li><code>p</code> 和 <code>q</code> 均存在于给定的二叉树中。</li>
</ul>
 
 
 
---
 
[提交记录](https://leetcode.cn/problems/lowest-common-ancestor-of-a-binary-tree/submissions/) | [题解](https://leetcode.cn/problems/lowest-common-ancestor-of-a-binary-tree/solution/)

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