Rolling Hash interview questions
17 rolling hash problems tagged across recent interview reports. Drilled most heavily by autodesk, capital one, and coupang.
Rolling Hash is a technique that computes polynomial hashes of substrings in linear time by reusing previous hash values, avoiding expensive substring comparisons. With 17 problems tagged across the pattern, it shows up heavily at Autodesk, Capital One, and Coupang. It's the backbone of pattern matching, longest repeating substring, and duplicate detection problems. The pattern matters because brute-force substring equality checks will TLE on medium to large inputs. StealthCoder solves rolling hash variants instantly when you hit one in a live assessment.
Most-asked rolling hash problems
| # | Problem | Diff | # Companies | Pass % |
|---|---|---|---|---|
| 01 | Shortest Palindrome | HARD | 5 | 41% |
| 02 | Count Prefix and Suffix Pairs II | HARD | 4 | 27% |
| 03 | Number of Distinct Substrings in a String | MEDIUM | 4 | 65% |
| 04 | Number of Subarrays That Match a Pattern I | MEDIUM | 4 | 67% |
| 05 | Repeated DNA Sequences | MEDIUM | 4 | 51% |
| 06 | Count Prefix and Suffix Pairs I | EASY | 2 | 78% |
| 07 | Find Beautiful Indices in the Given Array I | MEDIUM | 2 | 38% |
| 08 | Find Beautiful Indices in the Given Array II | HARD | 2 | 26% |
| 09 | Number of Subarrays That Match a Pattern II | HARD | 2 | 32% |
| 10 | Strings Differ by One Character | MEDIUM | 2 | 41% |
| 11 | Longest Duplicate Substring | HARD | 1 | 31% |
| 12 | Longest Repeating Substring | MEDIUM | 1 | 63% |
| 13 | Maximum Deletions on a String | HARD | 1 | 35% |
| 14 | Maximum Length of Repeated Subarray | MEDIUM | 1 | 51% |
| 15 | Minimum Time to Revert Word to Initial State I | MEDIUM | 1 | 41% |
| 16 | Minimum Time to Revert Word to Initial State II | HARD | 1 | 34% |
| 17 | Unique Substrings With Equal Digit Frequency | MEDIUM | 1 | 64% |
You can't drill every rolling hash variant before the assessment. StealthCoder runs invisibly during screen share and solves whichever variant they throw at you. No browser extension. No detection signature. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE.
Get StealthCoderRolling hash works by treating each substring as a polynomial evaluated at a base (usually a prime). Once you compute the hash for the first window, you slide it forward by subtracting the leftmost character's contribution, dividing by the base, multiplying by the base, and adding the new rightmost character. Recognition is usually triggered by problems asking for 'longest duplicate', 'count matching substrings', or 'find all occurrences of a pattern in linear time'. Common subtypes include fixed-window rolling hash (for subarray matching), variable-window (for longest repeating), and bidirectional variants (for prefix-suffix pairs). Drill order: start with fixed-window problems like maximum-length-of-repeated-subarray, move to longest-duplicate-substring, then tackle bi-directional variants like count-prefix-and-suffix-pairs-ii. StealthCoder is your edge when a hard rolling hash problem lands in your OA and you're unsure of base choice, modulo handling, or collision avoidance.
Companies that hire most on rolling hash
17 rolling hash problems.
You won't drill them all. Pass anyway.
Rolling Hash is one of the patterns interviews actually filter on. Memorizing every variant in a week is a fantasy. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds, no matter which rolling hash flavor lands in your live OA. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Rolling Hash interview FAQ
How many rolling hash problems should I drill before an OA?+
The pattern has 17 total problems. Aim to solve at least 6-8 across difficulty levels: 2-3 fixed-window, 2-3 longest-substring variants, and 1-2 bidirectional. Focus on recognition first, then hash implementation details.
Which companies drill rolling hash the hardest?+
Autodesk leads with 10 problems tagged on this pattern, followed by Capital One (7) and Coupang (6). If interviewing there, rolling hash is a major pattern. Palantir and Grammarly also weight it heavily.
How do I recognize a rolling hash problem in a real OA?+
Look for keywords: 'longest duplicate substring', 'find all occurrences', 'count substrings matching a pattern', 'repeated subarray', or 'prefix and suffix pairs'. If the naive approach is O(n^2) or O(n^2 log n) and you need O(n), rolling hash is likely the intended solution.
What's the most common pitfall in rolling hash implementations?+
Hash collisions and overflow. Always use a large prime modulus (like 10^9+7) and choose your base carefully (31 or 29 for strings). Test edge cases: single character, entire string, and wraparound behavior in your mod arithmetic.
Do I need to know rolling hash to pass most OAs?+
Not always. It's critical for companies like Autodesk and Capital One, but optional at many others. However, it's a fast skill to add: a few hours of drilling can flip a hard problem into seconds. Worth learning regardless of target company.