Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

May 2026

Summary

Coding round for a software engineer role at OpenAI. One question, but it had enough edge cases to keep me busy for a while. Solid problem that tests whether you actually think about encoding carefully or just slap a delimiter on it and call it done.

Questions Asked (1)

Design an encode and decode function pair for a list of strings. The encoded output must be a single string, it must be reversible for any possible input (including empty strings and strings with arbitrary characters), and you cannot rely on a special delimiter character that might appear in the data.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

My first instinct was to just join with a comma or pipe character and I nearly 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 as a decimal number followed by a delimiter (e.g., '#') and then the string itself. Since the length is known, the delimiter is unambiguous and the original strings can be recovered by reading the length, skipping the delimiter, and extracting exactly that many characters. This handles empty strings (length 0) and arbitrary characters, including the delimiter, because the length tells us exactly how many characters to read.

Pro tip: Mention that the length prefix must be parsed as a number, so the delimiter after the length is safe even if it appears in the string. Also, discuss trade-offs: this approach adds overhead proportional to the number of strings and their lengths, but it's simple and robust. For very large lists, consider a binary format with fixed-width integers for efficiency.

1. Clarify requirements and constraints

Confirm that the encoded output must be a single string, reversible for any input including empty strings and arbitrary characters, and cannot rely on a special delimiter that might appear in the data. Ask if there are any performance or size constraints.

2. Propose a length-prefix encoding scheme

Explain that you will encode each string as its length in decimal, followed by a delimiter (e.g., '#'), followed by the string itself. For example, ['hello', 'world'] becomes '5#hello5#world'. Emphasize that the delimiter is only used after the length, so it can appear in the string without ambiguity.

3. Detail the encode function

Iterate over the list, for each string compute its length, convert to string, append delimiter, then append the string. Concatenate all into one string. Handle empty list by returning empty string.

4. Detail the decode function

Parse the encoded string by reading digits until the delimiter to get the length, then skip the delimiter and extract exactly that many characters as the next string. Repeat until the end of the encoded string. Handle empty encoded string by returning empty list.

5. Discuss edge cases and trade-offs

Test with empty strings, strings containing the delimiter, strings with special characters, and empty list. Discuss overhead: length prefix adds O(log L) characters per string, which is acceptable. Mention alternative approaches like escaping or using a binary format, and their trade-offs.

Key Points to Mention

Length-prefix encoding avoids delimiter ambiguity because the length tells you exactly how many characters to read.
The delimiter (e.g., '#') is only used after the length, so it can safely appear in the string data.
Empty strings are encoded as '0#' and decoded correctly.
The encoded string is a single string, satisfying the requirement.
Time complexity: O(n) for both encode and decode, where n is total number of characters.
Space complexity: O(n) for the encoded string, with small overhead for length prefixes.
Alternative approaches: escaping special characters (but requires a delimiter that might appear), or using a binary format with fixed-width integers (more efficient but less human-readable).

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