232.用栈实现队列.py

#
# @lc app=leetcode.cn id=232 lang=python3
#
# [232] 用栈实现队列
#
# https://leetcode-cn.com/problems/implement-queue-using-stacks/description/
#
# algorithms
# Easy (57.87%)
# Total Accepted:    7.5K
# Total Submissions: 13K
# Testcase Example:  '["MyQueue","push","push","peek","pop","empty"]\n[[],[1],[2],[],[],[]]'
#
# 使用栈实现队列的下列操作：
#
#
# push(x) -- 将一个元素放入队列的尾部。
# pop() -- 从队列首部移除元素。
# peek() -- 返回队列首部的元素。
# empty() -- 返回队列是否为空。
#
#
# 示例:
#
# MyQueue queue = new MyQueue();
#
# queue.push(1);
# queue.push(2);
# queue.peek();  // 返回 1
# queue.pop();   // 返回 1
# queue.empty(); // 返回 false
#
# 说明:
#
#
# 你只能使用标准的栈操作 -- 也就是只有 push to top, peek/pop from top, size, 和 is empty
# 操作是合法的。
# 你所使用的语言也许不支持栈。你可以使用 list 或者 deque（双端队列）来模拟一个栈，只要是标准的栈操作即可。
# 假设所有操作都是有效的 （例如，一个空的队列不会调用 pop 或者 peek 操作）。
#
#
#


class MyQueue:

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

    def push(self, x: int) -> None:
        self.pushlist.append(x)
        """
        Push element x to the back of queue.
        """

    def pop(self) -> int:

        """
        Removes the element from in front of queue and returns that element.
        """
        if self.empty():
            return
        if self.poplist.__len__() == 0:
            pushlistsize = self.pushlist.__len__()
            for i in range(pushlistsize):
                self.poplist.append(self.pushlist.pop())
            return self.poplist.pop()
        else:
            return self.poplist.pop()

    def peek(self) -> int:
        """
        Get the front element.
        """
        if self.empty():
            return
        if self.poplist.__len__() == 0:
            pushlistsize = self.pushlist.__len__()
            for i in range(pushlistsize):
                self.poplist.append(self.pushlist.pop())
            return self.poplist[-1]
        else:
            return self.poplist[-1]

    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        if self.poplist.__len__()+self.pushlist.__len__() == 0:
            return True
        else:
            return False


# 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()