#
# @lc app=leetcode.cn id=145 lang=python3
#
# [145] 二叉树的后序遍历
#

# @lc code=start
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right


from typing import Optional, List

from utils import *


class Solution:
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        res = []
        stack = []
        if not root:
            return []
        stack.append(root)
        while stack:
            node = stack[-1]
            if node:
                stack.pop()
                stack.append(node)
                stack.append(None)
                if node.right:
                    stack.append(node.right)
                if node.left:
                    stack.append(node.left)
            else:
                stack.pop()
                node = stack.pop()
                res.append(node.val)
        return res


s = Solution()
s.postorderTraversal(generate_tree([1, None, 2, 3]))

# @lc code=end