QuickSort.cs

    public static void quickSort(int[] A, int left, int right)
    {
        if(left > right || left <0 || right <0) return; 

        int index = partition(A, left, right);

        if (index != -1)
        {
            quickSort(A, left, index - 1);
            quickSort(A, index + 1, right);
        }
    }

    private static int partition(int[] A, int left, int right)
    {
        if(left > right) return -1; 

        int end = left; 

        int pivot = A[right];    // choose last one to pivot, easy to code
        for(int i= left; i< right; i++)
        {
            if (A[i] < pivot)
            {
                swap(A, i, end);
                end++; 
            }
        }

        swap(A, end, right);

        return end; 
    }

    private static void swap(int[] A, int left, int right)
    {
        int tmp = A[left];
        A[left] = A[right];
        A[right] = tmp; 
    }