dsa8 min read
Number of Distinct Islands — DFS Shape Hashing
Count islands with unique shapes by encoding each DFS traversal path as a string and storing shapes in a set. A classic interview problem testing DFS + hashing.
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Hashing
5 articles
dsa11 min read
Rabin Karp Algorithm Explained — Rolling Hash for Pattern Matching Interviews
Rabin Karp uses a polynomial rolling hash to fingerprint each window of the text and matches against the pattern hash in expected O(n + m) time. Master this and you unlock substring search, plagiarism detection, and the entire family of hash-based string problems.
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dsa10 min read
Polynomial String Hashing — O(1) Substring Comparison with Prefix Hashes
Precompute polynomial prefix hashes once in O(n) and answer substring equality queries in O(1) for the lifetime of the string. The Swiss army knife behind longest duplicate substring, longest common substring binary search, and dozens of competitive programming techniques.
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dsa10 min read
Longest Common Substring — DP, Rolling Hash, and Suffix Array Approaches
Find the longest contiguous substring shared by two strings. The DP solution is O(n*m), binary search plus rolling hash gives O((n+m) log min(n,m)) expected, and suffix array plus LCP achieves O((n+m) log(n+m)).
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dsa5 min read
Find Duplicate Subtrees — LC 652 Serialize-and-Hash Pattern
Solve LeetCode 652 Find Duplicate Subtrees with postorder serialization and hashing — a Google and Amazon favorite. Includes Python and JavaScript solutions with O(n^2) and O(n) approaches.
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