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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

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).

思路：用一个queue来循环就可以了。

class MyStack {

    private Queue<Integer> queue;
    /** Initialize your data structure here. */
    public MyStack() {
        queue = new LinkedList<Integer>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        queue.add(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        int size = queue.size();
        int count = 0;
        while(count < size - 1) {
            queue.add(queue.poll());
            count++;
        }
        return queue.poll();
    }
    
    /** Get the top element. */
    public int top() {
        int size = queue.size();
        int count = 0;
        while(count < size - 1) {
            queue.add(queue.poll());
            count++;
        }
        // 选出来之后，再加入尾巴；
        int last = queue.poll();
        queue.offer(last);
        return last;
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue.size() == 0;
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */