[LeetCode][JavaScript]Binary Tree Postorder Traversal

 1 /**
 2  * Definition for a binary tree node.
 3  * function TreeNode(val) {
 4  *     this.val = val;
 5  *     this.left = this.right = null;
 6  * }
 7  */
 8 /**
 9  * @param {TreeNode} root
10  * @return {number[]}
11  */
12 var postorderTraversal = function(root) {
13     var stack = [], res = [], top;
14     if(root){
15         stack.push(root);
16     }
17 
18     while(stack.length !== 0){
19         top = stack.pop();
20         if(typeof top === "number"){
21             res.push(top);
22         }else{
23             stack.push(top.val);
24             if(top.right){
25                 stack.push(top.right);
26             }
27             if(top.left){
28                 stack.push(top.left);
29             }
30         }
31     }
32     return res;
33 };