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Grokking the Coding Interview Patterns in Go

Solution: Count the Number of Good Subsequences

Explore how to count the number of good subsequences in a string using dynamic programming and combinatorial math. This lesson helps you understand calculating combinations through factorials and modular inverses, optimizing through memoization. You will learn to handle frequency counts, binomial coefficients, and apply dynamic programming to improve efficiency and accuracy in solving subsequence problems.

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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.

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

Constraints:

  • 1≤1 \leqs.length ≤104\leq 10^4

  • s will only contain ...