using System;

public class Program

{

    public static void Main()

    {

                    // 74 is missing

            int[] arr = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 19, 20,

                        21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,

                        41, 42, 43, 44, 45, 46, 47, 48, 49, 18, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,

                        61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 50, 73, 72, 75, 76, 77, 78, 79, 80,

                        81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100 };

​

​

            #region Solution NO.1

​

            int sumNumbersInCurrentArr = 0;

            int sumNumbersInRang1To99 = 0;

​

            for (int i = 1; i < (arr.Length + 2); i++)

            {

                //calc sumNumbersInCurrentArr

                if (i <= arr.Length) sumNumbersInCurrentArr += arr[i - 1];

​

                //calc sumNumbersInRang1To99

                sumNumbersInRang1To99 += i;

            }

​

            Console.Write("");

​

            Console.Write($"--------------\n Solution No. #1 \n--------------\n missing item is : {sumNumbersInRang1To99 - sumNumbersInCurrentArr}");

​

            #endregion

​

            #region Solution No.2

            int missingNumber = 0;

​

            for (int i = 1; i < (arr.Length + 2); i++)

            {

                if (i <= arr.Length) missingNumber = missingNumber + arr[i - 1] - i;

​

                else missingNumber = missingNumber - i;

​

            }

​

            Console.Write($"\n\n\n--------------\n Solution No. #2 \n--------------\n missing item is : {~missingNumber + 1}\n\n");

​

            #endregion

    }

}