问题描述 后序遍历
代码：
# 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]:
result = []
def postorder(node):
if node:
postorder(node.left)
postorder(node.right)
result.append(node.val)
postorder(root)
return result运行结果：
