method1

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

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

            // TEMP variable for calculating. It means range necessary numbers.
            HashSet<int> range;

            // TEMP variable for current subsequence's length
            int _length;

            foreach (var number in uniqueNumbers)
            {
                bool containedLargeNumber = uniqueNumbers.Contains(number + 1);
                if (containedLargeNumber is false) continue;

                range = new HashSet<int>() { number, number + 1 };

                _length = nums.Count(_num => range.Contains(_num));

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

            return length;
        }