Microsoft·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Microsoft SWE interview with a pretty niche streaming/string processing problem. The question was technically specific and required knowing KMP or at least being able to reason about prefix-suffix overlap under time pressure.

Questions Asked (1)

You're receiving a stream of string chunks one at a time. Concatenate them and output every character that appears before a given stop token, but you can't buffer the whole stream first. The stop token might be split across two consecutive chunks. How do you implement this efficiently?

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The 'split across chunks' constraint is what makes this non-trivial.

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AI HintsAI Generated

Suggested Approach

Use a rolling buffer that holds only the last (stopTokenLength - 1) characters, since any match must complete within that window. Process each chunk by appending to the buffer, scanning for the stop token, and emitting all characters before the match (or all but the last stopTokenLength-1 characters if no match). This ensures O(n) time and O(m) space, where m is the stop token length.

Pro tip: Mention that you can optimize the search using the KMP algorithm or a rolling hash to avoid re-scanning the buffer, but for typical stop token lengths, a simple scan is sufficient. Also, clarify how you handle the stop token at the very end of the stream (e.g., if the stream ends without a full match, you must flush the remaining buffer).

1. Clarify requirements and constraints

Ask about the stop token length, whether it can be empty, and if the stop token itself should be excluded from output. Confirm that chunks arrive sequentially and that you cannot store the entire stream.

2. Design the rolling buffer

Maintain a buffer of size at most (stopTokenLength - 1) characters. This is the maximum overlap needed to detect a stop token split across chunks.

3. Process each chunk

Append the chunk to the buffer, then search for the stop token. If found, output all characters before the match and stop. If not found, output all characters except the last (stopTokenLength - 1) characters, and keep those in the buffer.

4. Handle stream termination

If the stream ends without finding the stop token, flush the remaining buffer to output. If the stop token is found, ensure no further chunks are processed.

5. Analyze complexity and edge cases

Discuss time complexity O(n) and space O(m). Cover edge cases: stop token longer than chunk, stop token at chunk boundary, empty stop token, and stop token not present.

Key Points to Mention

Rolling buffer of size stopTokenLength - 1 to handle split tokens
Efficient search within buffer (e.g., using KMP or simple scan)
Output characters only before the stop token, excluding the token itself
Handling stream termination and flushing remaining buffer
Time and space complexity analysis
Edge cases: empty stop token, stop token longer than chunk, multiple occurrences

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