String Algorithms and Pattern Matching Questions

Covers algorithmic techniques and practical skills for solving string problems and pattern matching tasks. Core algorithm knowledge includes substring search and pattern matching algorithms such as Knuth Morris Pratt, Rabin Karp, Boyer Moore, Z algorithm, Aho Corasick for multiple pattern matching, and rolling hash methods. Data structures and suffix structures are important, including tries, suffix arrays, suffix trees, and suffix automata, together with longest common prefix arrays and related construction techniques. Also includes dynamic programming approaches for string problems such as edit distance and longest common subsequence, palindrome and anagram detection methods, and regular expression concepts and engine behavior. Emphasizes algorithmic complexity analysis, time and space trade offs, memory and streaming constraints, and optimization strategies for very long inputs and high throughput text processing. Practical considerations include parsing and string manipulation idioms in common languages, Unicode and character encoding issues, edge case handling, test case design for strings, and real world applications such as log analysis, text search, and data transformation.

HardSystem Design

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You have millions of short patterns (indicators of compromise) and must match them against high-volume streaming logs under strict RAM limits. Propose a scalable architecture: discuss compressed automata (double-array trie), Bloom filters for pre-filtering, memory-mapped automata, and incremental updates. Detail trade-offs between false positives, memory, and update latency.

HardTechnical

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Explain SA-IS (Suffix Array Induced Sorting) algorithm for linear-time suffix array construction: describe S/L classification, LMS-substrings, induced sorting, and recursion. Explain why SA-IS achieves O(n) and discuss practical implementation challenges and when you'd prefer SA-IS over doubling algorithms.

HardTechnical

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Given a precomputed suffix array SA and LCP array for string s, describe how to find all occurrences of a pattern p using binary search on SA and using LCP values to accelerate comparisons. Provide complexity analysis and detail how to obtain occurrence positions sorted by original index.

MediumTechnical

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Given a string s, implement longest_repeated_substring(s: str) -> str using suffix array and LCP array. For the interview, it's acceptable to build suffixes and sort them (O(n log n)), compute adjacent LCPs, and return the longest repeated substring. Discuss complexity and how to improve to near-linear construction for large inputs.

MediumTechnical

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Write a Python function count_palindromic_substrings(s: str) -> int that counts palindromic substrings using the center-expansion technique (O(n^2) worst case). Explain the algorithm and briefly discuss Manacher's algorithm and when you would prefer it (O(n) time).

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