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Meta·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Meta SWE coding round focused on debugging a friend recommendation validator. Pretty straightforward premise but there were enough edge cases to keep you honest.

Questions Asked (1)

Q1

You're given a buggy function that validates a list of recommended friends for a user. Fix it so it correctly rejects recommendations that include the user themselves, existing friends, or duplicate user IDs.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The bug itself wasn't hard to spot once I actually read the tests.

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AI HintsAI Generated

Suggested Approach

First, clarify the function's contract and edge cases (e.g., empty list, null inputs). Then, walk through the buggy code to identify logical errors, and propose a fix using a set for O(1) lookups to efficiently check for self, existing friends, and duplicates. Finally, discuss trade-offs and test with representative cases.

Pro tip: Mention that you'd add unit tests covering edge cases like empty recommendations, all duplicates, and mixed invalid entries to ensure robustness. Also, consider if the function should return a boolean or a filtered list, and confirm with the interviewer.

1. Understand requirements and edge cases

Ask clarifying questions about input types, expected output, and constraints. Identify edge cases such as empty list, null values, and large inputs.

2. Analyze the buggy code

Trace through the existing function with sample inputs to pinpoint where it fails to reject self, existing friends, or duplicates.

3. Design an efficient solution

Use a set for existing friends and a set to track seen recommendations, enabling O(1) checks. Iterate through the list, rejecting invalid entries.

4. Implement and test

Write the corrected function, then test with cases like self-recommendation, existing friend, duplicate, and valid recommendations.

5. Discuss trade-offs and optimizations

Consider time/space complexity, alternative approaches (e.g., sorting), and whether to return a boolean or filtered list.

Key Points to Mention

  • Use a set for O(1) membership checks to avoid O(n^2) time complexity.
  • Handle edge cases: empty list, null inputs, and all invalid entries.
  • Clarify whether the function should return a boolean or a filtered list.
  • Consider space-time trade-offs: sets require extra space but improve speed.
  • Write unit tests to validate the fix against various scenarios.
  • Discuss potential follow-ups, such as handling large datasets or streaming input.

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