Two coding test questions related to dynamic programming:

• One from strings, and one from arrays.

Each question is worth 30 points.

The total test time is 1 hour, with each question having a 30-minute time limit.

A subsequence of a string is a new string that is formed by deleting some (or none) of the characters from the original string, without changing the order of the remaining characters.

A special subsequence of a string is a subsequence that has the following properties:

• It has a length of exactly K.
• It contains all unique characters.
• The sum of the frequencies of all the characters in the subsequence is the maximum possible sum among all special subsequences of length K.

Given a string S and an integer K, find the number of distinct special subsequences of the string. Since the number of special subsequences can be large, print the answer modulo 10^9 + 7.