public static int activityNotifications(List<int> expenditure, int d)
{
    var totalNotifications = 0;
    int[] frequency = new int[201]; // Create a frequency array.

    if (expenditure.Count > d)
    {
        for (int i = 0; i < d; ++i)
        {
            frequency[expenditure[i]]++;
        }

        for (int i = d; i < expenditure.Count; i++)
        {
            var median = GetMedian(frequency, d);

            if (expenditure[i] >= 2 * median)
            {
                totalNotifications++;
            }

            frequency[expenditure[i - d]]--; // Remove element from the frequency.
            frequency[expenditure[i]]++; // Add element to the frequency.
        }
    }

    return totalNotifications;
}

public static double GetMedian(int[] frequency, int d)
{
    var sum = 0;
    double median = 0;

    if (d % 2 == 1)
    {
        var mid = d / 2;
        for (int i = 0; i < frequency.Length; ++i)
        {
            sum += frequency[i];
            if (sum > mid)
            {
                median = i;
                break;
            }
        }
    }
    else
    {
        var mid1 = d / 2 - 1;
        var mid2 = d / 2;
        int median1 = -1, median2 = -1;

        for (int i = 0; i < frequency.Length; ++i)
        {
            sum += frequency[i];
            if (sum > mid1 && median1 == -1)
            {
                median1 = i;
            }
            if (sum > mid2)
            {
                median2 = i;
                median = (median1 + median2) / 2.0;
                break;
            }
        }
    }

    return median;
}