225. 用队列实现栈 - 力扣（LeetCode） (leetcode-cn.com)https://leetcode-cn.com/problems/implement-stack-using-queues/用一个队列就可以，pop的时候把队列里除了最后一个之外的所有元素重新加入队列，再poll()就好了。

class MyStack {

    Queue<Integer> q;

    public MyStack() {
        q = new LinkedList<Integer>();
    }

    public void push(int x) {
        q.add(x);
    }

    public int pop() {
        int size = q.size();
        for (int i=0;i<q.size()-1;i++){
            q.add(q.poll());
        }
        return q.poll();
    }

    public int top() {
        int t = this.pop();
        this.push(t);
        return t;
    }

    public boolean empty() {
        return q.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();
 */