CSharpQuickSort

public static int[] QuickSort(int[] arr)
{
  if (!arr.Any()) return new int[] { };

  var p = arr.First();
  var xs = arr.Skip(1);

  var lesser = xs.Where(x => x < p).ToArray();
  var greater = xs.Where(x => x >= p).ToArray();
  return (QuickSort(lesser).Concat(new[] { p }).Concat(QuickSort(greater))).ToArray();
}