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Apple·Machine Learning Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026

Summary

Phone screen for an ML engineer role at Apple. One coding problem, interval-merging flavored, which sounds straightforward until you're actually implementing it under pressure.

Questions Asked (1)

Q1

Given a string and a list of patterns, return the string with all matching substrings wrapped in bold HTML tags. Overlapping or adjacent matches should be merged into a single bold segment.

Algorithms & Data Structures
Author's notes

My first instinct was to just find all matches and slap tags around them, which obviously breaks the moment two patterns overlap.

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AI HintsAI Generated

Suggested Approach

Clarify requirements first, then propose an efficient algorithm using a trie or Aho-Corasick for multi-pattern matching, followed by interval merging to handle overlaps and adjacency. Discuss time/space complexity and edge cases, and consider practical optimizations for large-scale ML text processing.

Pro tip: Mention that this is a classic multi-pattern matching problem and that Aho-Corasick is the optimal solution for large pattern sets, showing you understand scalability beyond brute force.

1. Clarify requirements and edge cases

Ask about pattern overlap rules, case sensitivity, empty patterns, and whether patterns can contain special characters. Confirm that adjacent matches (e.g., 'ab' and 'bc' in 'abc') should be merged.

2. Choose the right algorithm

For a small number of patterns, a simple scan with each pattern is fine; for many patterns, use Aho-Corasick to find all matches in O(n + m + k) time. Explain the trade-offs.

3. Find all match intervals

Use the chosen algorithm to collect start and end indices of every occurrence of every pattern in the string. Store them as intervals [start, end).

4. Merge overlapping and adjacent intervals

Sort intervals by start index, then iterate and merge if the next interval's start is <= current end (adjacent or overlapping). This yields disjoint bold segments.

5. Construct the output string

Build the result by inserting <b> and </b> tags around each merged interval, ensuring no nested or duplicate tags. Return the final string.

Key Points to Mention

  • Aho-Corasick algorithm for efficient multi-pattern matching
  • Interval merging to handle overlaps and adjacency
  • Time and space complexity analysis (e.g., O(n + m + k) for Aho-Corasick)
  • Edge cases: empty string, no matches, patterns longer than string, case sensitivity
  • Handling special characters and HTML escaping if needed
  • Scalability considerations for large text corpora in ML pipelines

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