Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026

Summary

Got a coding question at OpenAI for a software engineer role that was deceptively simple on the surface but had a lot of edge case landmines buried in it. The kind of problem where you think you're done and then realize you've built something that breaks on its own test cases.

Questions Asked (1)

Design and implement serialize and deserialize functions for a string-to-string map, without using any built-in serialization libraries, such that the format is fully invertible even when keys and values contain arbitrary characters like colons, commas, or pipes.

System DesignAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

The naive instinct is to just join everything with some delimiter and call it done.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then propose a length-prefixed encoding scheme where each key and value is preceded by its length and a delimiter. Walk through the serialization and deserialization logic, emphasizing how the format remains unambiguous even with arbitrary characters.

Pro tip: Mention that the length prefix must be encoded in a way that itself is unambiguous, such as using a fixed-width integer or a delimiter that cannot appear in the length digits. Also, discuss the trade-off between human readability and robustness.

1. Clarify requirements and constraints

Ask about expected input size, performance needs, and whether the serialized format needs to be human-readable. Confirm that keys and values can contain any characters, including delimiters.

2. Choose an encoding scheme

Propose a length-prefixed format: for each key-value pair, write the length of the key, a delimiter, the key, the length of the value, a delimiter, and the value. Ensure the length encoding itself is unambiguous.

3. Implement serialize

Iterate over the map, and for each entry, append the length of the key, a separator (e.g., ':'), the key, the length of the value, a separator, and the value. Use a consistent separator that won't appear in the length digits.

4. Implement deserialize

Parse the serialized string by reading the length, then the separator, then exactly that many characters for the key, and similarly for the value. Repeat until the string is exhausted.

5. Test with edge cases

Verify round-trip correctness with keys and values containing delimiters, empty strings, and large inputs. Also test error handling for malformed input.

Key Points to Mention

Length-prefixed encoding avoids ambiguity with arbitrary characters.
Use a delimiter that cannot appear in the length field, such as a non-digit character.
Consider using a fixed-width integer for lengths to simplify parsing.
Ensure deserialization validates input and handles malformed data gracefully.
Discuss time and space complexity: O(n) for both operations, where n is the total size of keys and values.
Mention alternative approaches like escaping delimiters and their trade-offs.

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