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Openai·Software Engineer·Technical Phone Screen·Senior

Senior
May 2026

Summary

OpenAI software engineering interview focused on a pretty deep optimization problem around an IPv4 iterator. One question but they really wanted you to think through every layer of it.

Questions Asked (1)

Q1

You have an IPv4 iterator class that supports forward and backward iteration over a CIDR range, returning addresses as strings. How would you optimize it for both time and space complexity?

Algorithms & Data StructuresTechnical Trade-offsSystem Design
Author's notes

This one took me a minute to get fully into.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the current implementation's bottlenecks, then propose optimizations that reduce both time and space complexity, such as using integer arithmetic instead of string manipulation and lazy evaluation. Emphasize trade-offs and justify your choices based on typical usage patterns.

Pro tip: Mention that returning strings is inherently space-inefficient; suggest offering an option to return integers or a lightweight view, and only convert to string when needed. This shows you think about API design and real-world usage.

1. Identify inefficiencies

Analyze the current iterator: likely converts IP to string on each iteration, stores state as strings, and may precompute all addresses. Point out time spent on string formatting and space used for storing strings.

2. Optimize state representation

Represent the current IP as a 32-bit integer (or two 16-bit integers) and the range bounds as integers. This reduces space and makes increment/decrement O(1) arithmetic operations.

3. Lazy evaluation and on-demand conversion

Generate addresses on-the-fly without precomputing the entire range. Convert to string only when the iterator's next() or previous() is called, and consider caching or reusing string buffers if performance critical.

4. Handle edge cases and bidirectional iteration

Ensure correct behavior at range boundaries (e.g., network and broadcast addresses) and support both forward and backward iteration efficiently by adjusting the integer state accordingly.

5. Discuss trade-offs and alternatives

Compare with alternative designs: returning integers instead of strings, using a generator, or providing a method to get the next IP as an integer. Highlight the trade-off between API simplicity and performance.

Key Points to Mention

  • Use integer arithmetic for IP addresses instead of string manipulation to reduce time and space overhead.
  • Implement lazy iteration to avoid storing the entire range in memory.
  • Convert to string only when necessary, and consider reusing string buffers or using efficient formatting.
  • Maintain O(1) space complexity by storing only the current position and bounds.
  • Ensure bidirectional iteration works correctly with integer state, handling overflow/underflow at boundaries.
  • Discuss the trade-off between returning strings (user-friendly) and integers (performance), and suggest configurable output.

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