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Solution: Count the Number of Good Subsequences

Explore how to calculate the number of good subsequences in a string by leveraging dynamic programming and binomial coefficient techniques. Understand how to efficiently compute factorials and modular inverses to handle combination calculations. This lesson helps you master optimal solutions for subsequence problems using C++.

Statement

Count and return the number of good subsequences in the given string s. You may return the modulo 109+710^9 + 7 of the count.

  • A subsequence is a sequence formed from another sequence by deleting some or no elements while keeping the order of the remaining elements unchanged.

  • good subsequence is a subsequence of a string if it is not empty and the frequency of each character is the same.

Constraints:

  • 11 \leqs.length 104\leq 10^4

  • s will only contain ...