# Find longest substring with at least k repeats ### Longest substring where each character appears at least *k* times You are given: - a string `s` consisting of lowercase English letters - an integer `k` Return the **length of the longest 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 `-1`. #### Notes / assumptions - A substring is contiguous. - If `k <= 1`, the answer is `len(s)`. - You should handle edge cases like empty string, `k > len(s)`, and repeated characters. #### Example - `s = "aaabb", k = 3` → longest valid substring is `"aaa"` → return `3`. ### Constraints & Assumptions - Preserve the scope, facts, inputs, and requested outputs from the prompt above. - If the prompt leaves a detail unspecified, state a reasonable assumption before relying on it. - Keep the answer interview-ready: concise enough to present, but concrete enough to implement or evaluate. ### Clarifying Questions to Ask - Clarify input sizes, value ranges, mutability, return format, and tie-breaking. - State the target time and space complexity before coding. - Call out edge cases such as empty inputs, duplicates, invalid values, overflow, and boundary sizes. ### What a Strong Answer Covers - A clear algorithm with the right data structures and enough pseudocode or code-level detail to implement it. - A correctness argument that explains why the algorithm covers all required cases. - Time and space complexity, plus at least one alternative approach when relevant. - Focused tests for normal cases, edge cases, and failure modes. ### Follow-up Questions - How would the approach change if the input were streaming or too large for memory? - What invariants would you assert in production code? - Which tests would catch off-by-one, duplicate, or tie-breaking bugs?