C. Tyler and Strings（组合数学,树状数组维护前缀和）(Codeforces Round #775 (Div. 1, based on Moscow Open Olympiad i)

对我来说比较困难的一题了,尝试着自己写了一下,调不出来遂放弃.
Codeforces Round #775 (Div. 1, based on Moscow Open Olympiad in Informatics)
https://codeforces.com/contest/1648/problem/C
C.Tyler and Strings
题意:给你字符串 s , t s,t s,t,你可以对 s s s任意排序,问你有多少种方案使得 s s s重排后字典序会比 t t t小.
思路:枚举每次在哪里断掉.比如说在 i i i的位置,令 s j < t j s_j