（LeetCode）两个队列来实现一个栈

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<Integer> queue1 = new LinkedList<Integer>();
	LinkedList<Integer> queue2 = new LinkedList<Integer>();
    // 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<length-1;i++)
        {
        	queue2.offer(queue1.poll());
        }
        queue1.poll();
    	}
    	else
    	{
    		int length = queue2.size();
    	for(int i =0;i<length-1;i++)
    	{
    		queue1.offer(queue2.poll());
    	}
    	 queue2.poll();
    	}
    }
    // Get the top element.
    public int top() {
    	if(queue1.size()!= 0)
    	{
    		int length = queue1.size();
        for(int i =0;i<length-1;i++)
        {
        	queue2.offer(queue1.poll());
        }
        int result =  queue1.element();
        queue2.offer(queue1.poll());
		return result;
    	}
    	else
    	{
    		int length = queue2.size();
    	for(int i =0;i<length-1;i++)
    	{
    		queue1.offer(queue2.poll());
    	}
    	int result = queue2.element();
    	queue1.offer(queue2.poll());
    	return result;
    	}
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return queue1.size()==0&&queue2.size()==0;
    }

}