出栈操作

 public int pop() {
        if(empty()) {
            return -1;//两个队列都为空，意味着当前的栈为空
        }
        if(!qu1.isEmpty()) {
            int size = qu1.size();
            for (int i = 0; i < size-1; i++) {
                //for (int i = 0; i < qu1.size()-1; i++) {
                int val = qu1.poll();
                qu2.offer(val);
            }
            return qu1.poll();
        }else {
            int size = qu2.size();
            for (int i = 0; i < size-1; i++) {
                int val = qu2.poll();
                qu1.offer(val);
            }
            return qu2.poll();
        }
    }