给定一个二叉树，返回它的 后序 遍历。

示例:

输入: [1,null,2,3]
1

2
/
3

输出: [3,2,1]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
非递归
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if(root == null) return list;
        Stack<TreeNode> stack1 = new Stack<>();
        Stack<TreeNode> stack2 = new Stack<>();
        stack1.push(root);
        while(!stack1.isEmpty()){
            TreeNode node = stack1.pop();
            stack2.push(node);
            if(node.left != null){
                stack1.push(node.left);
            }
            if(node.right != null){
                stack1.push(node.right);
            }
        }

        while(!stack2.isEmpty()){
            list.add(stack2.pop().val);
        }
        return list;
    }
}