首发于 语雀文档@blueju

    重点：
    自顶向下

    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.  */
    15. function invertTree(root: TreeNode | null): TreeNode | null {
    16.     if (root === null) return null
    17.     const tempTreeNode = root.right
    18.     root.right = root.left
    19.     root.left = tempTreeNode
    20.     invertTree(root.left)
    21.     invertTree(root.right)
    22.     return root
    23. };