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.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

解析：java 中，Queue接口可使用的函数有 offer(E e)：添加到队列尾  peek():查看队列头  poll():删除队头元素 remove()移除队头元素

另外：Deque接口中的push(),pop()操作均是在队头操作。

代码如下：

class MyStack {

    // Push element x onto stack.

    Queue<Integer> q1=new LinkedList<Integer>();

    Queue<Integer> q2=new LinkedList<Integer>();

    public void push(int x) {

        q1.offer(x);

    }

    // Removes the element on top of the stack.

    public void pop() {

        while(q1.size()>1) q2.offer(q1.poll());

        q1.poll();

        Queue<Integer> q=q1;

        q1=q2;

        q2=q;

    }

    // Get the top element.

    public int top() {

      while(q1.size()>1) q2.offer(q1.poll());

      int x=q1.poll();

      q2.offer(x);

      Queue<Integer>q=q1;

      q1=q2;

      q2=q;

      return x;

    }

    // Return whether the stack is empty.

    public boolean empty() {

        return q1.isEmpty();

    }

}

　　运行结果：