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Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
May 2026

Summary

Interviewed for a software engineering role at OpenAI and got a coding question that looked deceptively simple on the surface. Basically had to build a mini in-memory database from scratch, which ended up being way more involved than I expected once edge cases started piling up.

Questions Asked (1)

Q1

Design and implement an in-memory database class supporting set(key, val) and get(key, sort_by=...) operations. Handle edge cases like key overwrites, missing keys, sorting on multi-field records, and stable ordering when sort values are equal. Also discuss your data structure choices and time complexity.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs
Author's notes

I started with a plain dict and felt good about it for about three minutes.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then propose a hash map for O(1) set/get and a sorting mechanism that preserves insertion order for ties. Implement get with optional sort_by, using a stable sort on the specified field, and discuss trade-offs between sorting on read vs. maintaining sorted indices.

Pro tip: Mention that Python's sorted() is stable, so you can sort by the key and rely on insertion order for ties—this avoids extra tie-breaking logic. Also, consider caching sorted results if get with the same sort_by is frequent.

1. Clarify requirements and edge cases

Ask about expected data types, whether sort_by can be multiple fields, and how to handle missing keys (return None or raise exception). Confirm that overwrites should update the value and possibly the insertion order.

2. Choose core data structures

Use a hash map (dict) for O(1) set and get by key. For sorting, either sort on read or maintain a secondary index; discuss trade-offs.

3. Implement set and get with sorting

For set, update the dict and track insertion order (e.g., using a counter or OrderedDict). For get with sort_by, retrieve all values, sort by the given field using a stable sort, and return the sorted list.

4. Handle edge cases

Ensure missing keys return a default or raise an error as specified. For multi-field records, sort_by should accept a field name or a tuple of fields. Stable ordering for equal sort values is naturally handled by stable sort.

5. Analyze time and space complexity

Set is O(1) average. Get without sort is O(1). Get with sort is O(n log n) due to sorting, where n is number of records. Space is O(n) for storage.

Key Points to Mention

  • Hash map provides O(1) average time for set and get by key.
  • Sorting on read is O(n log n) but simple; maintaining a sorted index can improve read performance at the cost of write complexity.
  • Stable sort ensures that records with equal sort values retain insertion order.
  • Overwriting a key should update the value and may update insertion order if using an ordered structure.
  • Missing keys: decide between returning None, raising KeyError, or a default value.
  • Multi-field sorting can be supported by allowing sort_by to be a tuple of field names.

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