Find longest substring with ≥k repeats
Company: TikTok
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: hard
Interview Round: Technical Screen
## Problem
You are given:
- a string `s` (consisting of lowercase English letters), and
- an integer `k`.
Return the length of the **longest contiguous substring** of `s` such that **every distinct character** in that substring appears **at least `k` times** within the substring.
If no such substring exists, return `0`.
## Input / Output
- **Input:** `s`, `k`
- **Output:** an integer length
## Constraints (typical interview bounds)
- `1 <= |s| <= 10^4`
- `1 <= k <= |s|`
- `s` contains only `'a'`–`'z'`
## Examples
1. `s = "aaabb"`, `k = 3` → output `3` (substring: `"aaa"`)
2. `s = "ababbc"`, `k = 2` → output `5` (substring: `"ababb"`)
3. `s = "abcd"`, `k = 2` → output `0`
Quick Answer: This question evaluates proficiency with string-processing techniques and character frequency analysis, focusing on algorithmic skills in the Coding & Algorithms domain. It is commonly asked to assess algorithmic reasoning about constraint handling, edge cases, and efficiency, and is positioned at the practical application level rather than purely conceptual understanding.