Day17 - 225. 用队列实现栈

题目链接：https://leetcode.cn/problems/implement-stack-using-queues/description/

1)用 collections.deque 实现队列

deque 是双端队列，适合高效的头尾操作：

class MyStack:

    def __init__(self):
        self.que = deque()

    def push(self, x: int) -> None:
        self.que.append(x)

    def pop(self) -> int:
        if self.empty():
            return None
        for i in range(len(self.que)-1):
            self.que.append(self.que.popleft())
        return self.que.popleft()

    def top(self) -> int:
        if self.empty():
            return None
        for i in range(len(self.que)-1):
            self.que.append(self.que.popleft())
        tmp = self.que.popleft()
        self.que.append(tmp)
        return tmp

        

    def empty(self) -> bool:
        return not self.que