* Definition for a binary tree node.
* function TreeNode(val) {
* this.left = this.right = null;
var postorderTraversal = function (root) {
// return postorderTraversal(root.left).concat(postorderTraversal(root.right)).concat(root.val);
let p = root; // 标识元素,用来判断节点是否应该出栈
while (stack.length > 0) {
const top = stack[stack.length - 1];
top.right === p || // 子节点已经遍历过了
(top.left === null && top.right === null) // 叶子元素