9-2. 用两个队列实现栈

代码：

class Solution:
    from collections import deque
    def __init__(self):
        self.queue1, self.queue2 = deque(), deque()
    
    # 用两个deque实现push操作
    def push(self, x):
        self.queue2.append(x)
        while self.queue1:
            self.queue2.append(self.queue1.popleft())
        self.queue1, self.queue2 = self.queue2, self.queue1
    
    # 用一个deque实现push操作
    def push(self, x):
        self.queue1.append(x)
        for i in range(len(self.queue1)-1):
            self.queue1.append(self.queue1.popleft())
            
    def pop(self):
        return self.queue1.popleft()
    
    def top(self): # 取队首元素
        return self.queue1[0]
    
    def empty(self):
        return not self.queue1