Quicksort in-place implementation in c#

hackerrank-quicksort.cs

int partition(int[] arr, int si, int ei)
{
    var pivot = arr[ei];
    var pivotIndex = si;
    for (var i = si; i < ei; i++)
    {
        // swap if current is lesser than pivot
        if (arr[i] < pivot)
        {
            var t = arr[pivotIndex];
            arr[pivotIndex] = arr[i];
            arr[i] = t;
            pivotIndex++;
        }
    }

    // swap pivot
    arr[ei] = arr[pivotIndex];
    arr[pivotIndex] = pivot;

    Console.WriteLine(String.Join(" ", arr));

    if ((pivotIndex - si ) > 1)
    {
        partition(arr, si, pivotIndex-1);
    }

    if (( ei - pivotIndex) > 1)
    {
        partition(arr, pivotIndex + 1, ei);
    }

    return pivotIndex;
}