题目描述：

Given a binary tree, return the postorder traversal of its nodes' values.

思路：利用栈，以及头插法进行操作

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */import java.util.*;public class Solution {    public ArrayList
     
       postorderTraversal(TreeNode root) {        ArrayList
      
        list=new ArrayList<>();        Stack
       
         stack=new Stack();        if(root==null)             return list;        stack.push(root);        while(!stack.isEmpty()){            TreeNode node=stack.pop();            list.add(0,node.val);            if(node.left!=null)                stack.push(node.left);            if(node.right!=null)                stack.push(node.right);        }        return list;    }}