Longest Duplicate Substring

HardStringsbinary search on answer

Solve this the way you would in the interview: out loud, on a timer, with an AI interviewer asking follow-ups while you code. You get a scored report at the end.

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Problem

Given a string s, find the longest substring that occurs at least twice in s. The occurrences may overlap. If several different substrings share that maximum length, return the **lexicographically smallest** of them. If no substring occurs twice, return the empty string "". An O(n log n) expected-time approach is the goal.

Examples

Input: s = "banana"
Output: "ana"
"ana" appears at index 1 and at index 3 (overlapping). No length-4 substring repeats.
Input: s = "abcd"
Output: ""
All characters are distinct, so nothing repeats.
Input: s = "abcabcxyzxyz"
Output: "abc"
"abc" and "xyz" both repeat with length 3, and "abc" is lexicographically smaller.

Constraints

  • 2 <= s.length <= 3 * 10^4
  • `s` consists of lowercase English letters

What the interviewer grades

A correct solution is one of five scores. The report also grades problem understanding (did you clarify inputs and edge cases before coding), problem solving, communication while you work, code quality, and testing. See a sample report, or read up on the pattern in the lessons.

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