225_ImplementStackUsingQueues.py

# coding: utf8


"""
    题目链接: https://leetcode.com/problems/implement-stack-using-queues/description.
    题目描述:

    Implement the following operations of a stack using queues.

    push(x) -- Push element x onto stack.
    pop() -- Removes the element on top of the stack.
    top() -- Get the top element.
    empty() -- Return whether the stack is empty.
    Notes:
    You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and
    is empty operations are valid.
    Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque
    (double-ended queue), as long as you use only standard operations of a queue.
    You may assume that all operations are valid (for example, no pop or top operations will be called on an empty
    stack).
    Credits:
    Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

"""

from collections import deque


class MyStack(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.queue = deque()

    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: void
        """
        self.queue.append(x)

    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        if self.empty():
            return -1

        for _ in range(len(self.queue) - 1):
            self.queue.append(self.queue.popleft())

        return self.queue.popleft()

    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        if self.empty():
            return -1

        return self.queue[-1]

    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        return not self.queue

# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()