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Solution: Count the Number of Good Subsequences

Explore how to count the number of good subsequences in a string by applying dynamic programming and binomial coefficients. Understand the computation of factorials and modular inverses to efficiently calculate combinations. This lesson guides you through optimizing subsequence counting while managing time and space complexity using Go.

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 ...