首发于 语雀文档@blueju

    代码：

    1. /**
    2.  * Definition for a binary tree node.
    3.  * class TreeNode {
    4.  *     val: number
    5.  *     left: TreeNode | null
    6.  *     right: TreeNode | null
    7.  *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    8.  *         this.val = (val===undefined ? 0 : val)
    9.  *         this.left = (left===undefined ? null : left)
    10.  *         this.right = (right===undefined ? null : right)
    11.  *     }
    12.  * }
    13.  */
    14. function lowestCommonAncestor(root, p, q) {
    15.     const dfs = (root, p, q) => {
    16.         if (root === null) return root
    17.         if (root === p || root === q) return root
    18.         const left = dfs(root.left, p, q)
    19.         const right = dfs(root.right, p, q)
    20.         if (left == null && right == null) return null;
    21.         if (left == null) return right;
    22.         if (right == null) return left;
    23.         if (left != null && right != null) return root
    24.     }
    25.     return dfs(root, p, q)
    26. };