Coding questions · C3 AI

Find longest substring with at least k repeats

This interview question evaluates algorithm design, data structures, correctness, complexity, edge cases, and implementation details in a realistic interview setting. A strong answer for Find longest substring with at least k repeats states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.

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.

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?

Longest Substring with Each Character at Least K Repeats

Hints

Choose a representation that makes the requested operation direct.
Handle empty inputs and boundary cases first.

Solution

Solution Alignment

The prompt asks for an implementation-level answer. The safest way to present it is to define the state, maintain clear invariants, then walk through complexity and tests.

Problem Restatement

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.

Recommended Approach

Choose traversal based on the required output. DFS is natural for subtree computations, reconstruction, and range pruning; BFS is natural for level order or side views. Keep per-depth or per-position state when the output depends on columns, rows, or depths.

Correctness

The implementation should maintain an invariant after each loop or operation that directly matches the problem statement. At termination, that invariant implies the returned value has considered every valid candidate exactly once, or has preserved the required data-structure state after every API call.

Complexity

Most tree traversals are O(n) time and O(h) recursion stack for DFS or O(w) queue space for BFS, where h is height and w is maximum width.

Edge Cases and Tests

Empty tree, one node, skewed tree, duplicate values when reconstruction assumes uniqueness, deep recursion, and tie-breaking for same row/column nodes.