Longest Repeated Substring with Rolling Hashes

Find the longest substring that occurs at least twice in a string (occurrences may overlap). A brute-force check is O(n³). Combine:

• binary search on the length L (if some substring of length L repeats, so does one of length L − 1), and
• a Rabin–Karp rolling hash to find repeats of length L in O(n), verifying candidate matches by direct comparison so hash collisions can't give wrong answers.