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Duolingo·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

Interviewed for a software engineering role at Duolingo and got a sorting problem that looked deceptively simple but had a few layers to it. The multi-key sort angle is what made it interesting.

Questions Asked (1)

Q1

Given a list of word-translation hint objects (each with a source word, candidate translations, a score, language pair, and category), sort them by score descending, then source word alphabetically, then by the length of the first candidate translation ascending.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The negating the score trick for descending sort is something I knew but fumbled explaining out loud.

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AI HintsAI Generated

Suggested Approach

Clarify the sorting requirements and edge cases, then propose a comparator-based approach that handles all three keys in order. Discuss the time complexity and whether a stable sort is needed, and consider if any optimizations or alternative data structures are relevant.

Pro tip: Mention that you would use a stable sort to preserve the original order for ties beyond the specified keys, and that you would write unit tests for edge cases like empty lists or missing translations.

1. Clarify requirements and edge cases

Ask about the expected input size, whether the list can be empty, if candidate translations can be empty, and if the sort should be stable. Confirm the exact sorting order and tie-breaking rules.

2. Design the comparator

Define a comparator that first compares scores in descending order, then source words alphabetically, then the length of the first candidate translation ascending. Handle cases where candidate translations list is empty.

3. Analyze complexity and choose algorithm

Discuss that comparison-based sorting is O(n log n) and that the comparator runs in O(1) time (assuming string comparisons are constant time). Mention that a stable sort like Timsort (Python's default) is suitable.

4. Implement and test

Write clean code using the language's built-in sort with a custom key or comparator. Test with edge cases: empty list, equal scores, equal source words, empty candidate translations, and varying lengths.

5. Discuss trade-offs and optimizations

Consider if the list is huge and memory is a concern, or if we can precompute the length of the first candidate translation to avoid repeated access. Mention that if the list is already partially sorted, adaptive sorts perform better.

Key Points to Mention

  • Comparator-based sorting with multiple keys
  • Stability of sorting algorithms and its importance for tie-breaking
  • Time complexity O(n log n) and space complexity O(n) for stable sort
  • Handling edge cases such as empty candidate translations or missing fields
  • Language-specific implementation details (e.g., Python's sorted with key function, Java's Comparator)
  • Testing strategy including unit tests for all sorting criteria

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