static int? OddOccurrencesInArray(int[] a)
{
    var count = new Dictionary<int, int>();

    foreach (var value in a)
    {
        if (!count.ContainsKey(value))
        {
            count[value] = 1;
        }
        else
        {
            count.Remove(value);
        }
    }

    return count.Count > 0 ? count.Keys.First() : (int?)null;
}