栈顶元素

public int top() {
        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();
            int val = -1;
            for (int i = 0; i < size; i++) {
                val = qu1.poll();
                qu2.offer(val);
            }
            return val;
        }else {
            int size = qu2.size();
            int val = -1;
            for (int i = 0; i < size; i++) {
                val = qu2.poll();
                qu1.offer(val);
            }
            return val;
        }
    }