C#

    class Test
    {
        public int NotContains(int[] array)
        {
            int count = 0;
            if (array == null) return 1;

            array = array.Where(x => x > 0).OrderBy(x => x).ToArray();

            foreach (var item in array.Length.GetCollection())
            {
                if (array[count++] != item)
                {
                    return item;
                }
            }
            return 0;
        }
    }
   static class test2
   {
        public static IEnumerable<int> GetCollection(this int x)
        {
            for (int i = 0; i < x - 1 ; i++)
            {
                yield return i;
            }
        }
   }