递归和循环

 1 # Definition for a binary tree node.
 2 # class TreeNode:
 3 #     def __init__(self, x):
 4 #         self.val = x
 5 #         self.left = None
 6 #         self.right = None
 7 
 8 class Solution:
 9     # @param {TreeNode} root
10     # @return {integer}
11     def maxDepth(self, root): 
12         if root == None: # 重要的是终点
13             return 0
14         return max (self.maxDepth(root.left),  self.maxDepth(root.right)) + 1 # self指向自己的指针，很重要