How do you find the longest consecutive sequence in an unsorted array?

Insert all elements into a hash set for O of one lookups. For each element, check if it is the start of a sequence by verifying that element minus one is not in the set. If it is a start, count consecutive elements by checking element plus one, plus two, and so on until the chain breaks. Track the maximum length found. This runs in O of n time because each element is visited at most twice: once during the outer loop and once during chain extension. The key insight is only starting chains from sequence beginnings, which prevents redundant work and keeps the total operations linear.