public class Solution {
    public int LongestConsecutive(int[] nums) {
        if (nums.Length < 2) return nums.Length;
        
        var set = new HashSet<int>(nums);
        var longest = 0;
        foreach (var n in set)
        {
            if (!set.Contains(n-1))
            {
                var length = 0;
                while (set.Contains(n+length))
                {
                    length++;
                    longest = Math.Max(longest, length);
                }
            }
        }
        
        return longest;
    }
}