Given a string, return the longest recurring substring.
This problem asks for the longest substring that appears more than once in the given string. A typical approach is to use suffix-based techniques, or binary search on the length combined with hashing/rolling hash to check whether a repeated substring of a given length exists. For smaller inputs, simpler brute-force or dynamic programming ideas may work, but the main challenge is comparing substrings efficiently and finding the maximum valid length.
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