Start by clarifying the problem: define 'substring' (contiguous) and 'repeating' (appears at least twice, possibly overlapping). Then present a solution using binary search on length combined with a rolling hash (Rabin-Karp) to achieve O(n log n) time, or suffix automaton for O(n) if optimal. Discuss trade-offs and handle edge cases.
Pro tip: Mention that you can use binary search on the answer because if a substring of length L repeats, then a substring of length L-1 also repeats (monotonic property). This shows deeper insight and can lead to an efficient solution.
Ask whether the substring must be contiguous, whether overlapping occurrences count, and what to return if no repeating substring exists. Confirm the expected time/space complexity.
Mention that checking all substrings would be O(n^3) or O(n^2) with hashing, which is inefficient for large n. This sets the stage for optimization.
Explain binary search on the length of the repeating substring, using a rolling hash to check for duplicates in O(n) per length, leading to O(n log n) overall. Alternatively, mention suffix automaton or suffix array for O(n) or O(n log n) solutions.
Describe how to compute rolling hashes for all substrings of a given length, store them in a hash set, and detect collisions. Discuss handling hash collisions (e.g., double hashing or verifying candidates).
State time and space complexity, and discuss edge cases like empty string, no repeating substring, and all characters identical. Mention that the answer could be the entire string if it repeats.
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