leetcode 225:Implement Stack using Queues

题目：

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).

分析：

本题是要用队列实现栈的相关操作，可以用两个队列来实现栈的四个操作，设定始终保证一个队列为空，两个队列分别为q1,q2：

1、push(x):

若q1非空，则x加入到q2中，然后将q1元素放入到q2中；若q2非空，则x加入到q1中，然后将q2元素放入到q1中.

2、pop():

若q1非空，则q1执行队列poll()操作；若q2非空，则q2执行队列poll()操作.

3、top():

若q1非空，则q1执行队列peek()操作；若q2非空，则q2执行队列peek()操作.

4、empty():

若队列q1和q2都为空，则返回true.

代码：

import java.util.ArrayDeque;
import java.util.Queue;
public class MyStack {
Queue<Integer> q1=new ArrayDeque<Integer>(); ;
Queue<Integer> q2=new ArrayDeque<Integer>();
// Push element x onto stack.
public void push(int x) {
if(q1.isEmpty()){
q1.offer(x);
while(!q2.isEmpty()){
int tmp=q2.peek();
q2.poll();
q1.offer(tmp);
}
}else{
q2.offer(x);
while(!q1.isEmpty()){
int tmp=q1.peek();
q1.poll();
q2.offer(tmp);
}
}
}

// Removes the element on top of the stack.
public void pop() {
if(!q1.isEmpty())
q1.poll();
if(!q2.isEmpty())
q2.poll();
}

// Get the top element.
public int top() {
if(!q1.isEmpty())
return q1.peek();
if(!q2.isEmpty())
return q2.peek();
return 0;
}

// Return whether the stack is empty.
public boolean empty() {
return q1.isEmpty()&&q2.isEmpty();
}
public static void main(String[] args){
MyStack stack=new MyStack();
boolean result=stack.empty();
System.out.println("empty:"+result);
}
}