From http://codereview.stackexchange.com/questions/52284/create-a-linq-killer-sudoku-cage-solver

Killer Sudoku Cage Solver

IEnumerable<IEnumerable<int>> GenerateCage(int num, int target, params int[] excluded)
{
    return GenerateCageInner(num, target, 1, 9, new HashSet<int>(excluded));
}

private IEnumerable<IEnumerable<int>> GenerateCageInner(int num, int target, int min, int max, HashSet<int> excluded)
{
    // Base case
    if (num == 0)
    {
        if (target == 0) yield return new int[0];
        yield break;
    }

    // Recursive case
    for (int i = min; i <= max && i <= target; i++)
    {
         if (excluded.Contains(i)) continue;

         foreach (var subsoln in GenerateCageInner(num - 1, target - 1, min + 1, max, excluded))
         {
             var soln = new List<int>();
             soln.Add(i);
             soln.AddRange(subsoln);
             yield return soln;
         }
    }
}