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.  */
    15. function postorderTraversal(root: TreeNode | null): number[] {
    16.     if (root === null)
    17.         return []
    18.     const postorder = []
    19.     const dfs = (root: TreeNode | null) => {
    20.         if (root === null)
    21.             return root
    22.         dfs(root.left)
    23.         dfs(root.right)
    24.         postorder.push(root.val)
    25.     }
    26.     dfs(root)
    27.     return postorder
    28. };