To prove safety Offer 09

 1 # -*- coding:utf-8 -*-
 2 class Solution:
 3     def __init__(self):
 4         self.list1 = []
 5         self.list2 = []
 6         
 7     def push(self, node):
 8         self.list1.append(node)
 9         # write code here
10     def pop(self):
11         if len(self.list2) == 0:
12             while len(self.list1) > 1:
13                 top = self.list1.pop(-1)
14                 self.list2.append(top)
15             return self.list1.pop(-1)
16         return self.list2.pop(-1)
17         # return xx