Problem: 225. 用队列实现栈

题目描述：

思路

1.对一个queue模拟栈的操作，同时用一个int类型的变量topElem记录每次每次队列队尾的元素（也即是模拟stack中的stack的栈顶元素）；
2.void push(int x):直接将元素入队，同时用topElem记录该元素；
3.int pop():先将队尾倒数两个元素前的元素出队再入队，再将topElem更新记录尾当前队列的队头元素，再将当前的对头元素出队、入队，最后将当前的对头元素出队返回
4.int top():直接返回topElem即可
5.boolean empty():直接判断队列是否位空即可

Code

class MyStack {
    Queue<Integer> queue;
    int topElem;

    public MyStack() {
        queue = new LinkedList<>();
        topElem = 0;
    }

    /**
     * Push the element to the queue
     *
     * @param x The element to be pushed
     */
    public void push(int x) {
        queue.offer(x);
        topElem = x;
    }

    /**
     * Remove the top element of stack
     *
     * @return int
     */
    public int pop() {
        int size = queue.size();
        //remain tow tail element of queue
        while (size > 2) {
            queue.offer(queue.poll());
            size--;
        }
        //recode the new tail element
        topElem = queue.peek();
        queue.offer(queue.poll());
        return queue.poll();
    }

    /**
     * Return the tail element of queue
     *
     * @return int
     */
    public int top() {
        return topElem;
    }

    /**
     * Check the empty
     *
     * @return boolean
     */
    public boolean empty() {
        return queue.isEmpty();
    }
}

/**
 * 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();
 */