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).
Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

思路：

• 题意是用队列数据结构来实现栈，要求只能使用Queue的基本函数，包括poll，isEmprty等
• 用两个队列pre和next来实现，pop（）操作是把一个非空队列复制到另一个空队列，最后一个不复制，每次判断哪个非空
• push（int x）的实现：先判断哪个队列非空，然后添加x，isEmpty（）是两个队列都为空的时候

-

代码：

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

class MyStack {
    Queue<Integer> pre = new LinkedList<Integer>();
    Queue<Integer> next = new LinkedList<Integer>();
    // Push element x onto stack.
    public void push(int x) {
        if(next != null){
            next.add(x);
        }else{
            pre.add(x);
        }
    }

    // Removes the element on top of the stack.
    public void pop() {
        if(!next.isEmpty()){
            while(!next.isEmpty()){
                int a = next.poll();
                if(!next.isEmpty()){
                    pre.add(a);
                }
            }
        }else{
            while(!pre.isEmpty()){
                int b = pre.poll();
                if(!pre.isEmpty()){
                    next.add(b);
                }
            }
        }

    }

    // Get the top element.
    public int top() {
        if(!next.isEmpty()){
            while(!next.isEmpty()){
                int c = next.poll();
                 pre.add(c);
                if(next.isEmpty()){
                   return c;
                }
            }
        }else{
            while(!pre.isEmpty()){
                int d = pre.poll();
                next.add(d);
                if(pre.isEmpty()){
                    return d;
                }
            }
      }
        return 0;
    }
    public boolean empty() {
        if(next.isEmpty() && pre.isEmpty()){
            return true;
        }else{
            return false;
        }
    }
}