static int PermMissingElement(int[] a)
{
    var sorted = (int[])a.Clone();
    Array.Sort(sorted);

    for (int k = 0; k < sorted.Length; k++)
    {
        if (sorted[k] != k + 1)
        {
            return k + 1;
        }
    }

    return sorted.Length + 1;
}