水题，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 = None class Solution(object): def maxDepth(self, root): """ :type root: TreeNode :rtype: int """ if root == None: return 0 else: left_depth = self.maxDepth(root.left) right_depth = self.maxDepth(root.right) return max(left_depth, right_depth)+1
