The core logic is straightforward, intersection by id, but the test coverage requirement is where they actually spend time with you.
Clarify assumptions about the User class and friend list properties, then choose an efficient algorithm using hash sets to find mutual friends in O(n+m) time. Implement the function with deduplication, and write comprehensive tests covering edge cases like empty lists, duplicate entries, and no mutual friends.
Pro tip: Mention that using sets automatically handles duplicates and provides O(1) lookups, but if memory is a concern, sorting and two-pointer approach can be used. Also, discuss the trade-off between modifying input lists versus creating new sets.
Ask if friend lists can contain duplicates, if order matters, and if the function should handle null inputs. Confirm the expected time and space complexity.
Decide to use hash sets for O(1) lookups and automatic deduplication. Consider converting the smaller friend list to a set for memory efficiency.
Write code that creates a set from one user's friends, then iterates through the other's friends, collecting those present in the set. Return the result as a list or set.
Include tests for empty friend lists, duplicate entries, no mutual friends, all mutual friends, and null inputs if applicable. Use assertions to verify correctness.
Explain that the solution runs in O(n+m) time and O(min(n,m)) space. Mention alternative approaches like sorting and two-pointer if memory is constrained.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.