Hard

145. Binary Tree Postorder Traversal

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ans = []
        if root == None:
            return ans
        self.helper(root, ans)
        return ans
    def helper(self, root, ans):
        if root == None:
            return
        self.helper(root.left, ans)
        self.helper(root.right, ans)
        ans.append(root.val)