![[agc23E]Inversionsdescriptionsolcode](/uploads/reation/bcimg7.png)
[agc23E]Inversionsdescriptionsolcode
Atcoderdescription给你\(n\)和\(\{a_i\}\)，你需要求所有满足\(p_i\le a_i\)的\(1-n\)排列的逆序对个数之和模\(10^9+7\)。\(n \le 2\times10^5\)sol首先考虑一下所有满足要求的排列总数。记\(cnt_i\)表示有多少个\(a_k\ge i\)，从大到小填数，方案数就是\[S=\prod_{i=1}^n...
