binary_tree_postorder_traversal.py

from typing import Optional, List


# 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


class Solution:
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        res = []

        def postorder(node):
            if not node:
                return []

            postorder(node.left)
            postorder(node.right)
            res.append(node.val)

        postorder(root)

        return res