using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        CombinationSum3(3, 7);

        Console.WriteLine("\n\n");

        CombinationSum3(3, 9);

    }

​

    static IList<IList<int>> CombinationSum3(int k, int n) {

        IList<IList<int>> results = new List<IList<int>>();

        Combination(k, n, 1, new List<int>(), results);

        foreach(var c in results)

            Console.WriteLine(string.Join(",", c.ToArray()));

        return results;

    }

    static void Combination(int k, int n, int start, List<int> combination, IList<IList<int>> result)

    {

        if(n == 0 && combination.Count == k)

        {

            result.Add(new List<int>(combination));

            return;

        }

        for(int i = start; i <= 9; i++)

        {    

            combination.Add(i);

            Combination(k, n - i, i + 1, combination, result);

            combination.RemoveAt(combination.Count - 1);

        }

    }

}