902 Numbers at most given digit set - wrote the code in the contest and first few hours after the content

902NumbersAtMostNGivenDigitSet.cs

public class Solution {
    public int AtMostNGivenDigitSet(string[] sortedDigits, int N)
        {
           var number = AtMostNGivenDigitSetHelper(sortedDigits, N);                    

           return number;           
        }

        public static int AtMostNGivenDigitSetHelper(string[] sortedDigits, int N)
        {
            if (N <= 0)
                return 0;

            var smallerEqualOnes = findLessEqual(sortedDigits);
            var smallerOnes = findLess(sortedDigits); 

            var hashset = new HashSet<int>();
            foreach (var item in sortedDigits)
            {
                hashset.Add((int)(item[0] - '0'));
            }

            // base case
            if (N < 10)
            {
                return smallerEqualOnes[N - 1];               
            }

            var count = 0;
            var chars = N.ToString();
            var leftmostDigit = (chars[0] - '0');
            var digits = sortedDigits.Length;
            var length = chars.Length;
            var numbers = smallerOnes[leftmostDigit - 1];

            count += numbers * (int)Math.Pow(digits, length - 1);

            if (hashset.Contains(leftmostDigit))
            {                
                // 123 - >= 100 and < 123
                count += AtMostNGivenDigitSetHelper(sortedDigits, Convert.ToInt32(chars.Substring(1)));
            }
                // < 100
                for(int i = 0; i < length - 1; i++)
                {
                    count += (int)Math.Pow(digits, i + 1);
                }
            

            return count;
        }

        private static int[] findLessEqual(string[] D)
        {
            var hashset = new HashSet<int>();
            foreach (var item in D)
            {
                hashset.Add((int)(item[0] - '0'));
            }

            var numbers = new int[9]; // 0 - 8 
            int count = 0;
            for (int i = 1; i <= 9; i++)
            {
                if (hashset.Contains(i))
                    count++;

                numbers[i - 1] = count;
            }

            return numbers;
        }

        // 1 2 3
        private static int[] findLess(string[] D)
        {
            var hashset = new HashSet<int>();
            foreach (var item in D)
            {
                hashset.Add((int)(item[0] - '0'));
            }

            var numbers = new int[9]; // 0 - 8 
            int count = 0;
            for (int i = 1; i <= 9; i++)
            {
                if (hashset.Contains(i - 1))
                    count++;

                numbers[i - 1] = count;
            }

            return numbers;
        }
}