leetcode 225. Implement Stack using Queues 双队列实现栈

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).

要求使用双队列实现一个栈的所有的功能，是一个很经典的问题，需要记住学习。

建议和这道题leetcode 232. Implement Queue using Stacks 双栈实现队列 一起学习。

1）取栈顶元素： 返回有元素的队列的首元素

2）判栈空：若队列a和b均为空则栈空

3）入栈：a队列当前有元素，b为空（倒过来也一样）则将需要入栈的元素先放b中，然后将a中的元素依次出列并入列倒b中。（保证有一个队列是空的）

4）出栈：将有元素的队列出列即可。

代码如下：

import java.util.LinkedList;import java.util.Queue;/* * 使用双队列来实现栈的功能 * （1）取栈顶元素： 返回有元素的队列的首元素 * （2）判栈空：若队列a和b均为空则栈空 * （3）入栈：a队列当前有元素，b为空（倒过来也一样）则将需要入栈的元素先放b中， * 然后将a中的元素依次出列并入列倒b中。（保证有一个队列是空的） * （4）出栈：将有元素的队列出列即可。 * */public class MyStack{    Queue<Integer> queue1=null;    Queue<Integer> queue2=null;    /** Initialize your data structure here. */    public MyStack()     {        queue1=new LinkedList<>();        queue2=new LinkedList<>();    }    /** Push element x onto stack. */    public void push(int x)     {        if(queue1.isEmpty())        {            queue1.add(x);            while(queue2.isEmpty()==false)                queue1.add(queue2.poll());        }else         {            queue2.add(x);            while(queue1.isEmpty()==false)                queue2.add(queue1.poll());        }    }    /** Removes the element on top of the stack and returns that element. */    public int pop()     {        if(queue1.isEmpty())            return queue2.poll();        else             return queue1.poll();    }    /** Get the top element. */    public int top()    {        if(queue1.isEmpty())            return queue2.peek();        else             return queue1.peek();    }    /** Returns whether the stack is empty. */    public boolean empty()    {        return queue1.isEmpty() && queue2.isEmpty();    }}/** * Your MyStack object will be instantiated and called as such: * MyStack obj = new MyStack(); * obj.push(x); * int param_2 = obj.pop(); * int param_3 = obj.top(); * boolean param_4 = obj.empty(); */