二叉树的最大深度

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

class Solution:
    def maxDepth(self, root):
        if not root:
            return 0
        max_left = self.maxDepth(root.left) 
        max_right = self.maxDepth(root.right) 
        return max(max_left, max_right) + 1