Find substring where all chars appear at least k
Company: TikTok
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: Medium
Interview Round: Technical Screen
You are given a string s and an integer k (1 ≤ k ≤ |s|). Return the length of the longest substring in which every distinct character occurs at least k times. If no such substring exists, return 0. Describe your algorithm, explain why it is correct, and analyze its time and space complexity. Discuss edge cases such as k = 1, k > |s|, and strings with all unique characters.
Quick Answer: This question evaluates proficiency in string processing, frequency analysis, and algorithmic reasoning for constrained-substring problems, along with the ability to analyze time and space complexity.