private static bool NextCombination(IList<int> num, int n, int k)
{
bool finished;
var changed = finished = false;

if (k <= 0) return false;
for (var i = k - 1; !finished && !changed; i--)
{
if (num[i] < n - 1 - (k - 1) + i)
{
num[i]++;
if (i < k - 1)
for (var j = i + 1; j < k; j++)
num[j] = num[j - 1] + 1;
changed = true;
}
finished = i == 0;
}
return changed;
}

private static IEnumerable Combinations<T>(IEnumerable<T> elements, int k)
{
var elem = elements.ToArray();
var size = elem.Length;