Solution: Find the Largest Number Possible

using System;
class Program
{
    /// <summary>
    /// Finds the largest number with the given number of digits and sum of digits.
    /// </summary>
    /// <param name="numberOfDigits">Number of digits.</param>
    /// <param name="sumOfDigits">Sum of digits.</param>
    /// <returns>The possible largest number.</returns>
    public static int[] FindLargestNumber(int numberOfDigits, int sumOfDigits)
    {
        // If the sum of digits is 0, then a number is possible only if the number of digits is 1.
        if (sumOfDigits == 0)
        {
            if (numberOfDigits == 1)
            {
                return new int[] { 0 };
            }
            else
            {
                return new int[] { -1 };
            }
        }
        // sumOfDigits is greater than the maximum possible sum.
        if (sumOfDigits > 9 * numberOfDigits)
        {
            return new int[] { -1 };
        }
        int[] result = new int[numberOfDigits];
        // Fill from most significant digit to least significant digit!
        for (int i = 0; i < numberOfDigits; i++)
        {
            // Place 9 to make the number largest
            if (sumOfDigits >= 9)
            {
                result[i] = 9;
                sumOfDigits -= 9;
            }
            // If remaining sum becomes less than 9, then fill the remaining sum
            else
            {
                result[i] = sumOfDigits;
                sumOfDigits = 0;
            }
        }
        return result;
    }
    // Driver code to test above function
    public static void Main(string[] args)
    {
        int sumOfDigits1 = 20;
        int numberOfDigits1 = 3;
        Console.WriteLine("[" + string.Join(", ", FindLargestNumber(numberOfDigits1, sumOfDigits1)) + "]"); // Output: 9, 9, 2
        int sumOfDigits2 = 100;
        int numberOfDigits2 = 2;
        Console.WriteLine("[" + string.Join(", ", FindLargestNumber(numberOfDigits2, sumOfDigits2)) + "]"); // Output: -1
    }
}