1 class Queue:
 2     def __init__(self):
 3         self.stack1 = []
 4         self.stack2 = []
 5 
 6     def push(self, node):
 7         self.stack1.append(node)
 8 
 9     def pop(self):
10         if not self.stack2:
11             while self.stack1:
12                 self.stack2.append(self.stack1.pop())
 
13             return self.stack2.pop()
14         return self.stack2.pop()

利用兩個棧實現隊列