Good Subsequences

Problem Overview

• Count good subsequences of a lowercase word where all characters in the subsequence occur equally often; the empty subsequence is not allowed.
• Input: a string of length 1–100000 over [a–z]. Output: number of good subsequences modulo 1,000,000,007.
• Real-world domain: string processing and combinatorics; typical coding interview problem.
• Company context: asked in Nutanix interviews and used as an interview question.