232_ImplementQueueUsingStacks.py

# coding: utf8


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

    Implement the following operations of a queue using stacks.

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

"""


class MyQueue(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.in_stack = []
        self.out_stack = []

    def push(self, x):
        """
        Push element x to the back of queue.
        :type x: int
        :rtype: void
        """
        self.in_stack.append(x)

    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        :rtype: int
        """
        if self.empty():
            return -1

        self.move()
        return self.out_stack.pop()

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

        self.move()
        return self.out_stack[-1]

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

    def move(self):
        if not self.out_stack:
            while self.in_stack:
                self.out_stack.append(self.in_stack.pop())

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