Python两个栈实现一个队列

 1 """
 2 用两个栈实现一个队列
 3 """
 4 class Solution:
 5     def __init__(self):
 6         self.stack1 = []
 7         self.stack2 = []
 8 
 9     def push(self, node):
10         # write code here
11         self.stack1.append(node)
12         
13     def pop(self):
14         # return xx
15         if self.stack1 == []:
16             return None
17         else:
18             for i in range(len(self.stack1)):
19                 self.stack2.append(self.stack1.pop())
20             out = self.stack2.pop()
21             for j in range(len(self.stack2)):
22                 self.stack1.append(self.stack2.pop())
23             return out