Array.Reverse()

            Random rnd = new Random();
            int tmp;
            int[] arr = new int[20];
            for (int i = 0; i < arr.Length; i++)
                arr[i] = rnd.Next(0, 21);
            for (int i = 0; i < arr.Length / 2; i++)
            {
                tmp = arr[i];
                arr[i] = arr[arr.Length - i - 1];
                arr[arr.Length - i - 1] = tmp;
            }