Method2

        public int FindLHS2(int[] nums)
        {
            int length = 0;

            // Getting all numbers in the array without diplicates
            HashSet<int> uniqueNumbers = new HashSet<int>(nums);

            // Stores the count of a number
            Dictionary<int, int> countNumber = new Dictionary<int, int>();

            // Counting the number of repetitions
            foreach (var number in uniqueNumbers)
            {
                countNumber[number] = nums.Count(_num => _num == number);
            }

            // Finding the longest combination
            int _length;
            foreach (KeyValuePair<int, int> repeats in countNumber)
            {
                _length = repeats.Value;

                _length += countNumber.GetValueOrDefault(repeats.Key + 1, 0);

                length = Math.Max(length, _length);
            }


            return length;
        }