Longest Duplicate Substring
A hard-tier problem at 31% community acceptance, tagged with String, Binary Search, Sliding Window. Reported in interviews at Coupang and 0 others.
Longest Duplicate Substring is a hard string problem that appears in assessments at Coupang and similar companies. You're asked to find the longest substring that appears at least twice in a given string. The catch: the naive approach (checking all substrings) times out instantly on large inputs. This is the kind of problem where candidates either know the pattern or blank entirely. If you hit it live and haven't drilled suffix arrays or rolling hash, you're in trouble. StealthCoder runs invisibly during your assessment and surfaces a working solution in seconds, so you move past it without the panic.
Companies that ask "Longest Duplicate Substring"
Longest Duplicate Substring is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage.
Get StealthCoderThe problem forces a choice between three viable approaches, each with different trade-offs. Suffix arrays give you the cleanest correctness guarantee but require implementation overhead. Rolling hash with binary search is faster and more practical for interviews, letting you binary search on substring length while hashing substrings in linear time. The trap: hash collisions will burn you if you pick weak constants or don't use modular arithmetic correctly. Sliding window alone won't cut it because you need to identify which substrings are actually duplicates, not just their positions. Most candidates attempt brute force first, see it TLE, and then switch mid-interview. Binary search plus rolling hash cuts through that mess. StealthCoder is your hedge if the implementation details derail you during the live OA.
Pattern tags
You know the problem.
Make sure you actually pass it.
Longest Duplicate Substring recycles across companies for a reason. It's hard-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Longest Duplicate Substring interview FAQ
Is this really asked in live coding rounds, or just LeetCode?+
Coupang reports it. It's a lower-frequency hard problem overall, but when it shows up in an OA, it tends to be a differentiation question. Companies use it to separate candidates who understand hashing and binary search from those who only know brute force.
What's the actual trick that makes this solvable?+
Binary search on the answer length, combined with rolling hash to detect duplicate substrings in linear time per candidate length. You're not searching for the substring itself. You're asking: 'Does a duplicate of length X exist?' and narrowing the range.
Can you solve this with just suffix arrays?+
Yes, but it's slower to code and debug in an interview. Suffix arrays are gold for multiple substring queries, but rolling hash plus binary search is more forgiving for a single query under time pressure.
What causes most submissions to fail?+
Hash collisions from weak constants or modular arithmetic mistakes. Candidates also often forget that you need to track actual substring positions, not just hashes. Off-by-one errors in the binary search bounds are common too.
How does this connect to the other string topics listed?+
Sliding window handles the substring extraction per candidate length. Binary search narrows the answer space. Rolling hash detects duplicates in O(n) time. String manipulation is the surface problem. Hashing and binary search are the depth that separates solved from TLE.
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