题目如下：

解题思路：凑数题+3，搞不懂为什么本题的难度是Hard，而是Medium。

代码如下：

# Definition for a binary tree node.# class TreeNode(object):#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = Noneclass Solution(object):    def postorderTraversal(self, root):        """        :type root: TreeNode        :rtype: List[int]        """        if root == None:            return []        res = []        stack = [root]        while len(stack) > 0:            node = stack.pop(0)            res.insert(0,node.val)            if node.left != None:                stack.insert(0,node.left)            if node.right != None:                stack.insert(0,node.right)        return res