Implement Stack using Queues

1 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).

题目出处：https://leetcode.com/problems/implement-stack-using-queues/

2 解题思路

1.题目的意思是使用队列实现栈的操作，这两个数据结构的主要区别在于一个将元素加入到尾部，一个将元素加入头部，因此考虑使用两个队列来处理这两种差异方式，始终保持一个队列为空，每次添加元素时，添加到空队列中，然后将非空队列添加到该队列中，并清空。

2.注意一下java中Queue的用法，参考网址：http://www.cnblogs.com/linjiqin/archive/2013/05/30/3107656.html

3 源代码

package com.larry.easy;

import java.util.Arrays;
import java.util.LinkedList;
import java.util.Queue;

/**
* 使用两个Queue来实现栈
* @author admin
*
*/
public class ImplementStackUsingQueues {
//始终保持一个队列为空；添加时，添加到空队列，然后将另一个加入
private Queue<Integer> que1 = new LinkedList<Integer>();//使用Queue时采用这种方式
private Queue<Integer> que2 = new LinkedList<Integer>();

// Push element x onto stack.
public void push(int x) {
if(que1.isEmpty()) {
que1.offer(x);
que1.addAll(que2);
que2.clear();
}else{
que2.offer(x);
que2.addAll(que1);
que1.clear();
}
}

// Removes the element on top of the stack.
public void pop() {
if(!que1.isEmpty()) que1.poll();
if(!que2.isEmpty()) que2.poll();
}

// Get the top element.

4000
public int top() {
if(!que1.isEmpty()) return que1.peek();
if(!que2.isEmpty()) return que2.peek();
return 0;
}

// Return whether the stack is empty.
public boolean empty() {
if(que1.isEmpty() && que2.isEmpty()) return true;
else return false;
}

public static void main(String[] args) {
ImplementStackUsingQueues isuq = new ImplementStackUsingQueues();
isuq.push(1);
isuq.push(2);
isuq.pop();
System.out.println(isuq.top());
}
}