Openai·Software Engineer·Technical Phone Screen·Senior

Senior

Apr 2026

Summary

OpenAI technical phone screen for a software engineering role, one coding question focused on iterator design. Pretty niche problem, not your typical LeetCode grind.

Questions Asked (1)

You're given an IPv4 iterator class with a next() method that returns IP addresses sequentially as strings with correct 32-bit wrap-around. Implement a prev() method for backward iteration, handling the lower address boundary and ensuring prev() followed by next() returns to the same position.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

This one tripped me up more than it should have.

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AI HintsAI Generated

Suggested Approach

First, clarify the iterator's internal representation and the expected behavior at boundaries. Then, design prev() by decrementing the address and handling wrap-around symmetrically to next(), ensuring that prev() followed by next() returns to the original position. Finally, discuss trade-offs such as using integer arithmetic versus string manipulation and potential edge cases.

Pro tip: Emphasize that the iterator's state should be a single integer, and both next() and prev() should update it atomically to avoid inconsistencies. Mention that testing with boundary values (0.0.0.0 and 255.255.255.255) is crucial.

1. Understand the existing iterator

Review the given next() method to see how it converts between string and integer representations and handles wrap-around. Identify the internal state variable and any assumptions.

2. Define prev() semantics

Determine that prev() should return the previous IP address in sequence, wrapping from 0.0.0.0 to 255.255.255.255. Ensure that calling prev() after next() (or vice versa) returns to the same address.

3. Implement prev() using integer arithmetic

Decrement the internal integer state by 1, applying modulo 2^32 to handle wrap-around. Convert the result back to the dotted-decimal string format.

4. Verify symmetry and edge cases

Test that prev() followed by next() returns the original address, and that boundary cases (0.0.0.0 and 255.255.255.255) behave correctly. Consider thread-safety if needed.

5. Discuss trade-offs and alternatives

Compare integer-based implementation with string manipulation, highlighting efficiency and simplicity. Mention potential issues with concurrent access and how to address them.

Key Points to Mention

Use a 32-bit unsigned integer to represent the IP address internally for easy arithmetic.
Apply modulo 2^32 when decrementing to handle wrap-around from 0.0.0.0 to 255.255.255.255.
Ensure that prev() and next() are inverses: prev() after next() returns the same address.
Convert between integer and dotted-decimal string using bitwise operations or standard library functions.
Consider edge cases: 0.0.0.0, 255.255.255.255, and multiple consecutive prev() calls.
Address thread-safety if the iterator may be used concurrently, e.g., using synchronization.

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