Microsoft·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Microsoft SWE interview, one technical round focused on a DNA sequence chaining problem. Pretty algorithmic, more depth than I expected for what seemed like a straightforward ordering task.

Questions Asked (1)

You're given a list of records, each with a start, end, and payload field. The records form a chain where one element's end matches the next element's start. Reorder them into a valid chain and return the concatenated payloads in order.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just sort, which would've been wrong.

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AI HintsAI Generated

Suggested Approach

Clarify assumptions (e.g., uniqueness of starts/ends, possibility of cycles) and then propose an efficient solution using a hash map to link records by their start/end values. Walk through the algorithm step-by-step, analyze time and space complexity, and discuss edge cases and potential trade-offs.

Pro tip: Mention that you would validate the input to ensure it forms a single chain (e.g., exactly one start with no incoming edge and one end with no outgoing edge) and handle cycles or disconnected components gracefully.

1. Clarify the problem

Ask questions to confirm constraints: Are all start/end values unique? Can there be cycles? Is the chain guaranteed to be valid? What should be returned if no valid chain exists?

2. Design the algorithm

Use a hash map to map each start value to its record. Identify the starting record (the one whose start is not any record's end). Then traverse the chain by following the end-to-start links, concatenating payloads.

3. Analyze complexity and edge cases

State that the solution runs in O(n) time and O(n) space. Discuss edge cases: empty list, single record, multiple chains, cycles, and duplicate start/end values.

4. Consider trade-offs

Compare with alternative approaches (e.g., sorting by start, topological sort) and explain why the hash map approach is optimal for this scenario. Mention memory vs. time trade-offs.

5. Test with examples

Walk through a simple example to verify correctness, and optionally mention how you would write unit tests for edge cases.

Key Points to Mention

Hash map for O(1) lookups to link records efficiently
Identifying the start of the chain by finding the record whose start is not any record's end
Handling cycles and disconnected components gracefully
Time and space complexity analysis (O(n) time, O(n) space)
Edge cases: empty input, single record, invalid chain
Trade-offs between different approaches (e.g., sorting vs. hashing)

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