Count number of inversions in an array in C#. An inversion in an array is when i<j but a[i]>a[j]. Running time of this algorithm is quadratic O(n*n).

InversionCounter.cs

public class InversionCounter
{
    int[] array;
    int inversion = 0;

    public InversionCounter (IEnumerable<int> arr)
    {
        array = arr.ToArray();
    }

    private int Merge(int firstStartIndex, int firstEndIndex,
                      int secondStartIndex, int secondEndIndex) 
    {
        int count = 0;
        for (int i = firstStartIndex; i <= firstEndIndex; i++)
        {
            for (int j = secondStartIndex; j <= secondEndIndex; j++)
            {
                if(array[i]>array[j])
                {
                    count++;
                }
            }
        }
        return count;
    }

    private int CountInversion(int start, int end) 
    {
        if(start >= end)
        {
            return 0;
        }

        int mid = start + (end-start)/2;

        return CountInversion(start, mid) + CountInversion(mid + 1, end)
            + Merge(start, mid, mid+1, end);
    }

    public int GetInversion() 
    {
        return CountInversion(0, array.Length-1);
    }
}