Openai·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Coding round at OpenAI for a software engineer role. One question, but it had enough moving parts to trip you up if you went in assuming it was trivial.

Questions Asked (1)

Design an encode and decode function pair for a list of strings. The encoding must handle arbitrary Unicode input without relying on any special delimiter characters.

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

My first instinct was to just join with some separator character, which is exactly the wrong move.

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AI HintsAI Generated

Suggested Approach

Start by clarifying constraints: the encoding must be self-delimiting and work with arbitrary Unicode, so avoid reserved delimiters. Propose a length-prefixed encoding (e.g., '<length>:<string>') and walk through encode/decode logic, handling edge cases like empty strings and multi-byte characters.

Pro tip: Mention that the length prefix should count Unicode code points or bytes consistently, and that using a fixed-width or variable-length integer with a clear separator (like ':') avoids ambiguity even if the string contains that separator.

1. Clarify requirements and constraints

Confirm that the encoding must be reversible for any Unicode string, cannot rely on special delimiters, and should be efficient. Ask about expected input size and performance needs.

2. Choose an encoding scheme

Propose a length-prefixed approach: for each string, output its length (in code points or bytes) followed by a separator like ':' and then the string itself. This is self-delimiting and delimiter-agnostic.

3. Define encode function

Iterate over the list, compute the length of each string (e.g., using len() for code points or byte length for UTF-8), and concatenate length + ':' + string. Return the combined string.

4. Define decode function

Parse the encoded string by reading digits until ':', convert to integer length, then extract exactly that many characters (or bytes) as the next string. Repeat until the end.

5. Discuss edge cases and trade-offs

Address empty strings, empty list, very large lengths, and Unicode normalization. Compare with alternatives like escaping or using a non-printable delimiter, and explain why length-prefixing is robust.

Key Points to Mention

Self-delimiting encoding: length prefix removes need for reserved delimiters.
Unicode handling: length can be in code points or bytes; be consistent and document choice.
Edge cases: empty strings, empty list, strings containing ':' or digits.
Efficiency: O(n) time and space, where n is total characters; avoid unnecessary copying.
Alternative approaches: escaping delimiters, using a delimiter unlikely to appear, or JSON-like encoding, and their trade-offs.
Decoding robustness: validate input format, handle malformed input gracefully.

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