232.用栈实现队列

链接

相当于用两个stack将队列的元素顺序颠倒了一遍。

class MyQueue:

    def __init__(self):
        self.stack_in = []
        self.stack_out = []

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

    def pop(self) -> int:
        if self.stack_out:
            return self.stack_out.pop()
        else:
            for i in range(len(self.stack_in)):
                self.stack_out.append(self.stack_in.pop())
            return self.stack_out.pop()

    def peek(self) -> int:
        res = self.pop()
        self.stack_out.append(res)
        return res

    def empty(self) -> bool:
        return not(self.stack_in or self.stack_out)

225. 用队列实现栈

链接

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
        else:
            for i in range(len(self.que) - 1):
                self.push(self.que.popleft())
            return self.que.popleft()

    def top(self) -> int:
        if self.empty():
            return None
        else:
            return self.que[-1]

    def empty(self) -> bool:
        return len(self.que) == 0