2539. Count the Number of Good Subsequences
A subsequence of a string is good if it is not empty and the frequency of each one of its characters is the same.
Given a string s, return the number of good subsequences of s. Since the answer may be too large, return it modulo 109 + 7.
A subsequence is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.