Given a string S, consider all duplicated substrings: (contiguous) substrings of S that occur 2 or more times. (The occurrences may overlap.)
Return any duplicated substring that has the longest possible length. (If S does not have a duplicated substring, the answer is "".)
Input: "banana" Output: "ana"
Input: "abcd" Output: ""
2 <= S.length <= 10^5
S consists of lowercase English letters.
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