1.是从1开始跳的，队列初始化为1

2.从1开始跳，for从2开始枚举

3.单调栈取等

while(d.size() > 1 && slope(d[1], d.front()) <= k(i)) d.pop_front();

while(d.size() > 1 && slope(d[d.size() - 2], d.back()) >= slope(d.back(), i)) d.pop

然后应该就能A啦