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Apple·Machine Learning Engineer·Technical Phone Screen·Senior

Senior
Jun 2026

Summary

Apple ML Engineer interview with a string pattern matching problem that sounds deceptively simple until you realize they want you to handle overlapping matches efficiently. The note about avoiding quadratic time is doing a lot of heavy lifting there.

Questions Asked (1)

Q1

Write a generator that scans a string and yields True once for each occurrence of a given pattern, including overlapping matches, without using a naive quadratic approach.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The overlapping part is what gets you.

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AI HintsAI Generated

Suggested Approach

Use the KMP algorithm to preprocess the pattern into a longest proper prefix-suffix (LPS) array, then scan the text in O(n+m) time. After each match, continue scanning from the next character (not skipping the pattern length) to capture overlapping occurrences. Yield True for each match found.

Pro tip: Emphasize that the LPS array allows the scan to resume without re-examining characters, and explicitly state that overlapping matches are handled by not advancing past the matched suffix. This shows you understand the algorithm's mechanics and can adapt it to the requirement.

1. Clarify requirements and constraints

Confirm that the pattern can be empty, the string length, and whether case sensitivity matters. Discuss expected time/space complexity and the need to handle overlapping matches.

2. Choose the algorithm

Select KMP for O(n+m) time, explaining why naive O(n*m) is unacceptable. Mention that KMP naturally supports overlapping matches by design.

3. Build the LPS array

Preprocess the pattern to compute the longest proper prefix that is also a suffix for each position. This enables efficient fallback during mismatches.

4. Scan the text and yield matches

Iterate through the string, using the LPS array to skip redundant comparisons. When a full match is found, yield True and set the pattern index to lps[pattern_index-1] to allow overlapping matches.

5. Test and analyze

Test with overlapping patterns (e.g., 'aa' in 'aaaa'), edge cases (empty pattern, no match), and analyze time/space complexity. Discuss trade-offs with other algorithms like Rabin-Karp.

Key Points to Mention

  • KMP algorithm and its O(n+m) time complexity
  • LPS (longest proper prefix-suffix) array construction and purpose
  • Handling overlapping matches by not resetting pattern index to 0 after a match
  • Generator implementation using yield to produce True lazily
  • Comparison with naive approach and why it's quadratic
  • Edge cases: empty pattern, pattern longer than text, no matches

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.