maxDepth.py

# 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 maxDepth(self, root):
        if not root:
            return 0
        else:
            return max(self.maxDepth(root.left), self.max(root.right)) + 1