Array.Sort()

            Random rnd = new Random();
            int tmp, maxIndex = 0;
            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; i++)
            {
                for (int k = i; k < arr.Length; k++)
                {
                    if (arr[k] > arr[maxIndex]) //change to if(arr[k] < arr[maxIndex]) to sort from small to big
                        maxIndex = k;
                }
                tmp = arr[i];
                arr[i] = arr[maxIndex];
                arr[maxIndex] = tmp;
            }

            //result example: 10, 9, 9, 7, 4, 3
            //result small to big: 3, 4, 7, 9, 9, 10