Stripe·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Stripe SWE screen, basically one coding question with some follow-up discussion on complexity and edge cases. Pretty straightforward but they pushed harder on the theory side than I expected.

Questions Asked (1)

Given a string containing letters mixed with '' and '&' characters, replace every '' with '0' and every '&' with '1' using the language's built-in string replace method. Then discuss the time complexity and how you'd handle edge cases like Unicode or repeated characters.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The replacement part took me about two minutes, nothing hard there.

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AI HintsAI Generated

Suggested Approach

Start by writing a clean solution using the language's built-in replace method, then analyze the time complexity as O(n) since each replacement scans the string. Finally, discuss edge cases like Unicode and repeated characters, explaining how the built-in method handles them and any potential pitfalls.

Pro tip: Mention that while built-in replace is efficient, it may not be suitable for very large strings due to memory allocation; in such cases, consider a streaming approach or StringBuilder. Also, clarify that Unicode characters are typically handled correctly if the string is treated as a sequence of code points, but be aware of surrogate pairs in some languages.

1. Clarify requirements and constraints

Ask about the expected input size, language, and whether the string is mutable or immutable. Confirm that only '*' and '&' need replacement and that other characters remain unchanged.

2. Implement using built-in replace

Use the language's string replace method (e.g., str.replace('*', '0').replace('&', '1') in Python) to perform the replacements. Ensure the order doesn't matter since the characters are distinct.

3. Analyze time and space complexity

Explain that each replace operation scans the string, so two passes result in O(n) time. Space complexity is O(n) due to creating a new string (in immutable languages).

4. Discuss edge cases

Address Unicode: built-in replace works on code units, so it handles Unicode correctly if the string is properly encoded. For repeated characters, the method replaces all occurrences by default. Mention potential issues with surrogate pairs if the language treats strings as UTF-16.

5. Consider alternatives and trade-offs

If performance is critical, discuss a single-pass approach using a character array or StringBuilder, which avoids multiple scans and reduces memory overhead. Compare readability vs. efficiency.

Key Points to Mention

Time complexity: O(n) for each replace, so O(n) overall since two passes.
Space complexity: O(n) for immutable strings; mutable strings can be O(1) extra if modified in place.
Built-in replace handles all occurrences, including repeated characters.
Unicode: strings are sequences of code points; replace works on code units, so it's safe for most cases but watch for surrogate pairs in UTF-16.
Edge cases: empty string, no '*' or '&', string with only '*' or '&'.
Trade-offs: built-in replace is concise but may be less efficient for very large strings; manual iteration can be more memory-efficient.

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