Rolling Hash is a powerful technique used to compute hash values for substrings efficiently as a window moves across a string. Instead of recalculating the hash from scratch for every substring, a rolling hash updates the value in constant time by removing the contribution of the outgoing character and adding the new one. This idea is widely used in substring search algorithms and pattern matching tasks.
In coding interviews, rolling hash often appears in problems involving duplicate substrings, pattern detection, and fast substring comparisons. It is a core component of the Rabin–Karp algorithm and is closely related to topics like String processing and String Matching. Understanding how hashing works and how collisions can occur is essential, which is why knowledge of Hash Function concepts is helpful.
Common patterns you will encounter include:
Mastering rolling hash helps you solve advanced string problems efficiently, especially when brute-force substring comparisons would be too slow.
Rolling hash operates directly on strings and substrings, so understanding string manipulation and indexing is essential.
Prefix hash arrays follow the same idea as prefix sums, allowing constant-time substring hash computation.
Rolling hash relies on polynomial hashing and modular arithmetic, which are core concepts of hash functions.
Rolling hash is often combined with sliding window techniques to update substring hashes efficiently.
| Status | Title | Video | Leetcode | Solve | Difficulty | Companies | Topics |
|---|---|---|---|---|---|---|---|
| 187. Repeated DNA Sequences | Solve | Medium | Amazon+1 | ||||
| 718. Maximum Length of Repeated Subarray | Solve | Medium | Indeed+2 | ||||
| 1062. Longest Repeating Substring | Solve | Medium | VMware | ||||
| 1461. Check If a String Contains All Binary Codes of Size K | Solve | Medium | Google | ||||
| 1554. Strings Differ by One Character | Solve | Medium | Airbnb+1 | ||||
| 1698. Number of Distinct Substrings in a String | Solve | Medium | Bloomberg+3 | ||||
| 2168. Unique Substrings With Equal Digit Frequency | Solve | Medium | Expedia | ||||
| 2261. K Divisible Elements Subarrays | Solve | Medium | Amazon+1 | ||||
| 3006. Find Beautiful Indices in the Given Array I | Solve | Medium | - | ||||
| 3023. Find Pattern in Infinite Stream I | Solve | Medium | - | ||||
| 3029. Minimum Time to Revert Word to Initial State I | Solve | Medium | - | ||||
| 3034. Number of Subarrays That Match a Pattern I | Solve | Medium | - | ||||
| 3291. Minimum Number of Valid Strings to Form Target I | Solve | Medium | - |
Start Easy, progress to Hard.
Frequently appear alongside Rolling Hash.
Common questions about Rolling Hash.
A rolling hash is a hashing technique that updates the hash value of a substring as the window moves across a string. Instead of recomputing the entire hash, it adjusts the previous value in constant time.
Collisions can be reduced by choosing a good base and modulus or by using double hashing with two different mod values. In critical cases, matches are verified with direct string comparison.
Rolling hash allows fast substring comparisons and efficient pattern searching. It is commonly used in problems involving duplicate substrings, pattern detection, and the Rabin–Karp algorithm.
Yes. The Rabin–Karp string searching algorithm uses rolling hash to compute hash values of substrings and quickly check potential matches against a pattern.
Practicing 20–30 well-chosen problems is usually enough to understand the core patterns. Focus on substring comparison, duplicate detection, and pattern matching scenarios.