[LeetCode]--225. 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.

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

就是两个queues，轮流把除最后一个之外的都放入另一个queue中。

class MyStack {
        Queue<Integer> q1 = new LinkedList<Integer>();
        Queue<Integer> q2 = new LinkedList<Integer>();


        // Push element x onto stack.
        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> temp = q1;
            q1 = q2;
            q2 = temp;
        }

        // Get the top element.
        public int top() {
            while (q1.size() > 1)
                q2.offer(q1.poll());
            int x = q1.poll();
            q2.offer(x);
            Queue<Integer> temp = q1;
            q1 = q2;
            q2 = temp;
            return x;
        }

        // Return whether the stack is empty.
        public boolean empty() {
            return q1.isEmpty();
        }
    }