原题例如以下：

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).

基本思路是：

如果有两个队列Q1和Q2，当二者都为空时。入栈操作能够用入队操作来模拟，能够随便选一个空队列，如果选Q1进行入栈操作。如今如果a,b,c依次入栈了（即依次进入队列Q1）。这时如果想模拟出栈操作，则须要将c出栈。由于在栈顶。这时候能够考虑用空队列Q2，将a，b依次从Q1中出队，而后进入队列Q2，将Q1的最后一个元素c出队就可以。此时Q1变为了空队列。Q2中有两个元素，队头元素为a。队尾元素为b。接下来如果再运行入栈操作，则须要将元素进入到Q1和Q2中的非空队列，即进入Q2队列，出栈的话，就跟前面的一样。将Q2除最后一个元素外所有出队，并依次进入队列Q1，再将Q2的最后一个元素出队就可以。

Java实现代码例如以下：

class MyStack {	LinkedList
     
       queue1 = new LinkedList
      
       ();	LinkedList
       
         queue2 = new LinkedList
        
         ();    // Push element x onto stack.    public void push(int x) {        if(queue1.size()==0&&queue2.size()==0)        {        	queue1.offer(x);        }        else if(queue1.size()==0)        {        	queue2.offer(x);        }        else        {        	queue1.offer(x);        }    }    // Removes the element on top of the stack.    public void pop() {    	if(queue1.size()!= 0)    	{    		int length = queue1.size();        for(int i =0;i