Implement Stack using Queues

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：用两个queue，q1, q2, q1用来装，然后当要pop的时候，把q1里面所有元素，except最后一个，全部放到q2里面，然后pop最后一个，然后q1，q2交换顺序。所有的费时操作都在pop和top中。

class MyStack {    Queue<Integer> queue1 = new LinkedList<Integer>();    Queue<Integer> queue2 = new LinkedList<Integer>();    // Push element x onto stack.    public void push(int x) {        queue1.add(x);    }    // Removes the element on top of the stack.    public void pop() {        if(queue1.size()!=0){            while(queue1.size()>1){                queue2.add(queue1.poll());            }            queue1.poll();            Queue<Integer> temp = queue1;            queue1 = queue2;            queue2 = temp;        }    }    // Get the top element.    public int top() {        while(queue1.size()>1){            queue2.add(queue1.poll());        }        return queue1.peek();    }    // Return whether the stack is empty.    public boolean empty() {        return (queue1.size()+queue2.size()) == 0;    }}