add: 二叉树的最近公共祖先

README.md

@@ -318,6 +318,11 @@ LeetCode 与 LintCode 解题记录。此为个人练习仓库，代码中对重
   - LeetCode 98. 验证二叉搜索树 https://leetcode-cn.com/problems/validate-binary-search-tree/
   - LintCode 95. 验证二叉查找树 https://www.lintcode.com/problem/validate-binary-search-tree/
 
+- [二叉树的最近公共祖先](src/tree/lowest-common-ancestor-of-a-binary-tree.js)
+
+  - LeetCode 236. 二叉树的最近公共祖先 https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-tree/
+  - LintCode 88. 最近公共祖先 https://www.lintcode.com/problem/lowest-common-ancestor-of-a-binary-tree/description
+
 - [另一个树的子树](src/tree/subtree-of-another-tree.js)
 
   - LeetCode 572. 另一个树的子树 https://leetcode-cn.com/problems/subtree-of-another-tree/

src/tree/lowest-common-ancestor-of-a-binary-tree.js

@@ -0,0 +1,20 @@
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @param {TreeNode} p
+ * @param {TreeNode} q
+ * @return {TreeNode}
+ */
+export const lowestCommonAncestor = function (root, p, q) {
+  if (!root || root === p || root === q) return root
+  const left = lowestCommonAncestor(root.left, p, q)
+  const right = lowestCommonAncestor(root.right, p, q)
+  if ((left === p && right === q) || (left === q && right === p)) return root
+  return left || right
+}

test/tree/lowest-common-ancestor-of-a-binary-tree.test.js

@@ -0,0 +1,8 @@
+import { lowestCommonAncestor } from '../../src/tree/lowest-common-ancestor-of-a-binary-tree'
+import Tree from './Tree'
+
+test('二叉树的最近公共祖先', () => {
+  const head = Tree.arrToTree([3, 5, 1, 6, 2, 0, 8, null, null, 7, 4])
+  expect(lowestCommonAncestor(head, head.left, head.right)).toEqual(head)
+  expect(lowestCommonAncestor(head, head.left, head.left.right.right)).toEqual(head.left)
+})