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