/**
/*** Definition for a binary tree node.* function TreeNode(val, left, right) {*     this.val = (val===undefined ? 0 : val)*     this.left = (left===undefined ? null : left)*     this.right = (right===undefined ? null : right)* }*/
/*** @param {TreeNode} root* @return {number[]}*/*/var postorderTraversal = function(root) {var retArr = []if (!root) return retArrvar helpFunc = (r)=>{if (!r) return helpFunc(r.left)helpFunc(r.right)retArr.push(r.val)}helpFunc(root)return retArr
}

用模拟栈来实现（方法一） 逆向

var postorderTraversal = function(root) {if(!root) return []let cur = rootvar stack = []var retArr = []while(stack.length !== 0 || cur){// 右节点入栈if (cur) {stack.push(cur)retArr.push(cur.val)cur = cur.right}   else {var tmp = stack.pop()cur = tmp.left}}return retArr.reverse()}

用模拟栈来实现（方法二 ） （单栈）

var postorderTraversal = function(root) {if(!root) return []let cur = rootvar stack = []var retArr = []var r = null // 记录访问过的子节点while(stack.length !== 0 || cur){// 左节点入栈if (cur) {stack.push(cur)cur = cur.left}   else {cur = stack[stack.length - 1] // 读取栈顶元素，非弹出// 如果右子树存在，并且没有被访问if (cur.right && cur.right !=r) {cur = cur.right} else {// 弹出并访问var tm = stack.pop()retArr.push(tm.val)r = curcur = null}}}return retArr}