Medium
Hashing & Frequency Maps
Longest Consecutive Sequence
Only start counting from a number that has no left neighbor.
Approach
Put everything in a set for O(1) lookups. For each value, only begin a run if value − 1 is absent (it's the start of a streak), then walk value+1, value+2, … counting length. Every element is visited at most twice, giving O(n) despite the nested look.
Time complexity
O(n)
Space complexity
O(n)
Common mistake
Sorting (O(n log n)) — acceptable, but it misses the intended O(n) set-based trick that starts only from run beginnings.
See it run, step by step
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