牛客IOI周赛20-普及组前三题题解 A:  https://blog.nowcoder.net/n/85365cbeb751407abc32e77ccf111164 B:  https://blog.nowcoder.net/n/49a08dfa345c42629a953ede5fbba9f2 C:  https://blog.nowcoder.net/n/2adbe0a98dab4973b01e6d2d5398be77