Microsoft·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Microsoft coding screen, one question that looked like a simple string processing problem until the edge cases started piling up. The stop-token-spanning-chunks thing is sneakier than it sounds.

Questions Asked (1)

Given a stream of string chunks, yield characters to the output until a stop token is detected. The stop token may be split across two consecutive chunks, so you need to buffer characters that could be the beginning of the stop token and only emit them once you've confirmed they aren't part of it.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I thought I had this immediately and started coding a simple scan.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints (e.g., stop token length, overlapping patterns, streaming nature) and then propose a solution using a buffer that holds a suffix of the processed data that could be a prefix of the stop token. Use a string matching algorithm like KMP to efficiently detect the stop token across chunk boundaries, and discuss trade-offs between simplicity and performance.

Pro tip: Mention that you would handle the case where the stop token itself contains overlapping prefixes (e.g., 'abab') by using a proper prefix function, and that you would test with edge cases like empty chunks or stop token at the very beginning.

1. Clarify requirements and constraints

Ask about the stop token's length, whether it can contain special characters, and if the stream is infinite or has a known end. Confirm that characters after the stop token should not be emitted.

2. Design the buffering strategy

Maintain a buffer of characters that could be the start of the stop token. After each chunk, append to the buffer and check for the stop token; if not found, emit characters that cannot be part of any future match.

3. Choose an efficient matching algorithm

Use KMP or a similar algorithm to avoid re-scanning the buffer, ensuring O(n) time complexity. Alternatively, for simplicity, use a naive approach if the stop token is short, but discuss the trade-off.

4. Handle edge cases and termination

Address cases where the stop token spans multiple chunks, the stream ends without a stop token, or the stop token appears at the boundary. Ensure the buffer is flushed appropriately when the stream ends.

5. Analyze complexity and trade-offs

Discuss time and space complexity: O(n) time with O(m) space where m is the stop token length. Compare with simpler approaches and justify your choice based on expected input size and performance needs.

Key Points to Mention

Buffer management: keep only the longest suffix that is a prefix of the stop token.
Use of KMP or prefix function to efficiently detect partial matches.
Handling of overlapping patterns within the stop token itself.
Edge cases: empty chunks, stop token at start, stream ending without stop token.
Time and space complexity analysis.
Trade-offs between naive and optimized approaches for different stop token lengths.

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