priority_queue會在插入時自動排序好，要取最大值就直接從top取

priority_queue說明：

https://dangerlover9403.pixnet.net/blog/post/197006355-%5B%E6%95%99%E5%AD%B8%5D-priority-queue

本題解答：

https://66lemon66.blogspot.com/2021/01/ac-0.html