Openai·Backend Engineer·Technical Phone Screen·Senior

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May 2026Remote

Summary

Interviewed for a backend role at OpenAI and got a classic encode/decode string problem. Nothing flashy, but it required more thought than I initially gave it credit for.

Questions Asked (1)

Design an encode function that serializes a list of strings into a single string, and a decode function that reconstructs the original list. The strings can contain any ASCII character, so you can't rely on a simple delimiter.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just join with a comma and I almost said it out loud before catching myself.

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AI HintsAI Generated

Suggested Approach

Use a length-prefix encoding: for each string, write its length followed by a delimiter (e.g., '#') and then the string itself. This ensures unambiguous decoding regardless of the characters in the strings. In the decode function, parse the length, skip the delimiter, and read exactly that many characters.

Pro tip: Mention that the length prefix must be in a fixed format (e.g., decimal digits) and that the delimiter is only needed to separate the length from the string, not to separate strings. Also, discuss handling edge cases like empty strings and empty lists.

1. Clarify requirements and constraints

Confirm that the input is a list of strings, output is a single string, and that strings can contain any ASCII character including digits and delimiters. Ask about expected input sizes and performance requirements.

2. Choose an encoding scheme

Decide on a length-prefix approach: encode each string as its length, a delimiter (e.g., '#'), and the string. Explain why a simple delimiter fails due to arbitrary characters.

3. Implement encode

Iterate through the list, for each string append str(len(s)) + '#' + s to the result. Handle empty strings (length 0) and empty list (return empty string).

4. Implement decode

Parse the encoded string: read digits until '#', convert to integer length, then read exactly that many characters as the next string. Repeat until the end of the encoded string.

5. Test and discuss trade-offs

Test with edge cases: empty list, strings with '#', digits, and special characters. Discuss time/space complexity (O(n) where n is total characters) and potential alternatives like escaping or using a different delimiter.

Key Points to Mention

Length-prefix encoding avoids ambiguity with arbitrary characters.
The delimiter (e.g., '#') separates the length from the string, not strings from each other.
Handle empty strings and empty lists correctly.
Time complexity is O(n) for both encode and decode, where n is total number of characters.
Space complexity is O(n) for the encoded string.
Consider robustness: what if the input is malformed? (e.g., invalid length prefix)

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