Find Longest Consecutive Sequence in an Unsorted?

Array

int LongestConsecutive(int[] nums)

HashSet<int> set = new HashSet<int>(nums);

int longest = 0;

foreach (int num in set)

if (!set.Contains(num - 1))

Follow on:

int currentNum = num;

int length = 1;

while (set.Contains(currentNum + 1))

currentNum++;

length++;

longest = Math.Max(longest, length);

return longest;

Explanation:

Check only starts of sequences, count consecutive numbers using HashSet for O(n).