BinaryTreePostorderTraversal.py

from typing import List
from collections import deque
# Definition for a binary tree node.
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution1:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        result = []
        self.helper(root, result)
        return result

    def helper(self, root, res):
        if root:
            self.helper(root.left, res)
            self.helper(root.right, res)
            res.append(root.val)


class Solution2:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        stack = [root]
        result = deque()
        while stack:
            current = stack.pop()
            result.appendleft(current.val)
            if current.left:
                stack.append(current.left)
            if current.right:
                stack.append(current.right)
        return list(result)