Leetcode: Implement Stack using Queues

mplement 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.

Understand the problem:
We can use two queues, queue1 and queue2.
 -- push(x), push x into queue1. Time complexity O(1)
 -- pop(), dequeue all n - 1 elements and enqueue into queue2. dequeue the last element. 
Swap queue1 and queue2. Time complexity O(n).
 -- top() dequeue all n - 1 elements and enqueue into queue2, dequeue the last element and save the as return, then enqueue it into queue2. Swap queue1 and queue2. Time complexity is O(n).
 -- empty(), check if queue1 is empty. 

Code (Java):

class MyStack {
    private Queue<Integer> queue1;
    private Queue<Integer> queue2;
    
    public MyStack() {
        this.queue1 = new LinkedList<Integer>();
        this.queue2 = new LinkedList<Integer>();
    }
    
    // Push element x onto stack.
    public void push(int x) {
        queue1.offer(x);
    }

    // Removes the element on top of the stack.
    public void pop() {
        while (queue1.size() > 1) {
            queue2.offer(queue1.poll());
        }
        
        queue1.poll();
        
        // Swap queue1 and queue2
        Queue<Integer> temp = queue1;
        queue1 = queue2;
        queue2 = temp;
    }

    // Get the top element.
    public int top() {
        while (queue1.size() > 1) {
            queue2.offer(queue1.poll());
        }
        
        int ret = queue1.poll();
        queue2.offer(ret);
        
        // Swap queue 1 and queue2
        Queue<Integer> temp = queue1;
        queue1 = queue2;
        queue2 = temp;
        
        return ret;
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return queue1.isEmpty();
    }
}