由数据范围反推算法复杂度以及算法内容
https://www.acwing.com/blog/content/32/

第七讲 时空复杂度分析
https://www.acwing.com/blog/content/3398/

C++ 手动开O2优化
https://blog.csdn.net/liyizhixl/article/details/77803291
https://blog.csdn.net/Tony_Wang_JT/article/details/104444969
#pragma GCC optimize(2)