Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jul 2026Remote

Summary

Meta SWE coding round with a string manipulation problem that looked easy at first glance but had some tricky requirements around deduplication and complexity analysis.

Questions Asked (1)

Given an array of strings (possibly with duplicates), return all words that appear as a substring of at least one other word in the array. The output should be deduplicated. Walk through a baseline approach and its complexity, then propose and implement an optimized solution. Include at least 5 test cases covering duplicates, identical strings, and a no-match scenario.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was the naive O(n^2 * L^2) brute force with nested loops and the built-in substring check, which I explained fine.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and edge cases, then present a baseline O(n^2 * L) approach using pairwise substring checks. Next, propose an optimized solution using a trie or sorting by length to efficiently find substrings, and implement it with clean code. Finally, walk through test cases covering duplicates, identical strings, and no matches.

Pro tip: Mention that sorting words by length allows early termination and avoids redundant checks, and that using a trie can reduce substring search time to O(total characters). Also, explicitly handle the case where a word appears multiple times but is not a substring of any other distinct word.

1. Clarify requirements and edge cases

Confirm that duplicates in input should be deduplicated in output, and that a word is not considered a substring of itself unless it appears in another word. Discuss handling of empty strings and case sensitivity.

2. Present baseline approach and complexity

For each word, check if it is a substring of any other word using nested loops. Time complexity O(n^2 * L) where n is number of words and L is average length; space O(n) for output.

3. Propose optimized solution

Sort words by length descending, then insert each word into a trie. For each word, search the trie for any word that contains it as a substring, or use a set of all substrings of longer words. Alternatively, use Aho-Corasick for multiple pattern matching.

4. Implement optimized solution

Write code that builds a trie of all words, then for each word, traverse the trie to check if it appears as a substring of any other word. Use a set to deduplicate results.

5. Test with comprehensive cases

Include test cases: (1) duplicates like ['a','a','b'] -> ['a'] if 'a' is substring of another? Actually need careful: if 'a' appears twice, is it substring of another? No, unless another word contains 'a'. So test with ['a','ab','abc'] -> ['a','ab']; (2) identical strings ['abc','abc'] -> [] because neither is substring of another distinct word; (3) no matches ['abc','def'] -> []; (4) empty string ['','abc'] -> ['']; (5) overlapping substrings ['ab','bc','abc'] -> ['ab','bc']; (6) case sensitivity ['a','A'] -> [].

Key Points to Mention

Time and space complexity trade-offs between baseline and optimized approaches
Use of trie or Aho-Corasick for efficient substring matching
Handling duplicates and deduplication using a set
Edge cases: empty strings, identical strings, no matches, case sensitivity
Sorting by length to reduce unnecessary comparisons
Early termination when a word is found as a substring

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