```
# Definition for a binary tree node
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
# @param root, a tree node
# @return an integer
def maxDepth(self, root):
def maxDepthHelper(root):
if not root: return 0
return max(1+maxDepthHelper(root.left), 1+maxDepthHelper(root.right))
return maxDepthHelper(root)
```