How to Solve Count K-Subsequences of a String With Maximum Beauty Problem

You are given a string s and an integer k. A k-subsequence is a subsequence of s, having length k, and all its characters are unique, i.e., every character occurs once. Let f(c) denote the number of times the character c occurs in s. The beauty of a k-subsequence is the sum of f(c) for every character c in the k-subsequence. For example, consider s = "abbbdd" and k = 2: Return an integer denoting the number of k-subsequences whose beauty is the maximum among all k-subsequences. Since the answer may be too large, return it modulo 109 + 7. A subsequence of a string is a new string formed from the original string by deleting some (possibly none) of the characters without disturbing the relative positions of the remaining characters. Notes