Hash Sets: Seen It Before?

A set answers "is this in the collection?" instantly. Use it to detect duplicates, find missing numbers and the longest consecutive run. Longest consecutive sequence [100, 4, 200, 1, 3, 2] → the longest run of consecutive numbers is 1, 2, 3, 4 → length 4. Sorting would be O(n log n); a set does it in O(n).