Find Maximal Product Number

Recently I was taking an online quiz and one of the code challenge is to find the maximal product number. I have came up with the below code with an algorithm. It took around 40 mins to write this so I accept it’s not a perfect code, but if anyone can improve it please do and let me know 🙂

        List<int> total = new List<int>();

        public int MaxAlgo(int input, string lpn)
        {
            int sum = 1, inp = input, mod;
            if (input == 0)
            {
                int num = Convert.ToInt32(lpn);

                while (num != 0)
                {
                    mod = num % 10;
                    num /= 10;
                    sum *= mod;
                }
                total.Add(sum);

            }
            for (int i = Math.Min(inp, input); i >= 1; i--)
            {
                MaxAlgo(input - i, lpn + "" + i);
            }
            return sum;
        }

        public int MaximalProduct(int input)
        {
            if (input <= 0)
            {
                return 0;
            }
            else
            {
                MaxAlgo(input, "");
                var rtnval = 0;
                for (int i = rtnval; i < total.Count; i++)
                {
                    rtnval = Math.Max(rtnval, total[i]);
                }
                return rtnval;

            }
        }

Test : call the function “MaximalProduct(8)” which should return 18.

Note: I converted a string to int32, so it may throw a ‘System.OverflowException’ if the number is > 11