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Confluent·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Confluent SWE interview with a ranking/voting algorithm problem that looks deceptively clean on the surface but has enough edge cases to keep you busy for a while.

Questions Asked (1)

Q1

You're given preference rankings from n users over m songs. Define a 'beats' relation where song x beats song y if more than half the users prefer x over y, or exactly half prefer x and x has a smaller ID. Then define popularity by how many songs each song beats, breaking ties by song ID. Return all songs sorted from most to least popular.

Algorithms & Data Structures
Author's notes

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Suggested Approach

Clarify the input format and constraints, then design an algorithm that computes pairwise comparisons efficiently. Use a tournament-style approach or sorting with a custom comparator, and handle tie-breaking rules explicitly.

Pro tip: Mention that the 'beats' relation may not be transitive, so sorting with a comparator is invalid; instead, compute all pairwise comparisons or use a more efficient method like merge sort with comparisons.

1. Clarify the problem

Ask about input format (e.g., list of rankings per user), constraints on n and m, and expected output format. Confirm tie-breaking rules.

2. Design the algorithm

Propose an O(m^2 * n) brute-force approach: for each pair of songs, count users preferring each, apply tie-break, and tally beats. Then sort by beats count and ID.

3. Optimize if needed

If m is large, consider using a more efficient method like merge sort with comparisons, but note that the relation may not be transitive, so sorting may not be valid. Alternatively, use bitsets to speed up comparisons.

4. Handle edge cases

Discuss cases where n is even/odd, ties in beats count, and songs with equal beats. Ensure tie-breaking by smaller ID is applied correctly.

5. Analyze complexity and test

State time and space complexity, and walk through a small example to verify correctness.

Key Points to Mention

  • The 'beats' relation is not necessarily transitive, so sorting with a comparator is invalid.
  • Brute-force pairwise comparison is O(m^2 * n) time, which may be acceptable for small m.
  • Tie-breaking: if exactly half prefer x over y, x beats y if x has smaller ID.
  • Popularity is the count of songs a song beats; sort descending by count, then ascending by ID.
  • Edge cases: n=0, m=1, all users have identical rankings, and ties in beats count.
  • Potential optimization: use bitsets to represent user preferences and count beats faster.

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