Use recursion can easy to solve this problem, like below:
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def postorderTraversal(self, root):
"""
:type root: TreeNode
:rtype: List[int]
"""
# left -> right -> root
if not root:
return []
return self.postorderTraversal(root.left) + self.postorderTraversal(root.right) + [root.val]But we have to use iteration to solve this problem, the stack and the hashmap is the solution