Find longest consecutive sequence in an unsorted array
Reported in Snap interview loops. Hash set based linear-time approach for sequence detection.
Interview scenario
Context for Snap candidates:
Return the length of the longest run of consecutive integers in any order.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at Snap: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
Insert all values into a hash set, then start counting only from numbers that have no predecessor (num - 1 absent). This prevents repeated work and ensures each sequence is expanded once.
For each valid start, advance while num + 1 exists and track maximum length. The average complexity is O(n), which is better than sorting-based O(n log n) for large input.
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