Leetcode Problem 2539. Count the Number of Good Subsequences

2539. Count the Number of Good Subsequences

Leetcode Solutions

Combination Based Approach with Mod Inverse

  1. Initialize a frequency array to count the occurrences of each character.
  2. Calculate the maximum frequency among all characters.
  3. Precompute factorials and modular inverses for numbers up to the maximum frequency.
  4. Iterate over all possible frequencies from 1 to the maximum frequency.
  5. For each frequency, calculate the product of 1 + binomial_coefficient(frequency of character, current frequency) for all characters.
  6. Subtract 1 from the product to exclude the empty subsequence.
  7. Add the result to the final answer, taking the modulo at each step.
  8. Return the final answer.
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Dynamic Programming with Combinatorics

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