Find Missing Numbers in a Sorted Array Without Inbuilt Functions

There are three methods as written below −

• In the first method

  Use the formula n(n+1)/2 that counts the number of elements and then need to subtract from the elements in the array.

• In the second method

  Create a new array and traverse through the entire array and make the number false whichever is found.

• In the third method

  Use Xor operation. which gives the missing number.

Example

 Live Demo

using System;
namespace ConsoleApplication{
   public class Arrays{
      public int MissingNumber1(int[] arr){
         int totalcount = 0;
         for (int i = 0; i < arr.Length; i++){
            totalcount += arr[i];
         }
         int count = (arr.Length * (arr.Length + 1)) / 2;
         return count - totalcount;
      }
      public int MissingNumber2(int[] arr){
         bool[] tempArray = new bool[arr.Length + 1];
         int element = -1;
         for (int i = 0; i < arr.Length; i++){
            int index = arr[i];
            tempArray[index] = true;
         }
         for (int i = 0; i < tempArray.Length; i++){
            if (tempArray[i] == false){
               element = i;
               break;
            }
         }
         return element;
      }
      public int MissingNumber3(int[] arr){
         int result = 1;
         for (int i = 0; i < arr.Length; i++){
            result = result ^ arr[i];
         }
         return result;
      }
   }
   class Program{
      static void Main(string[] args){
         Arrays a = new Arrays();
         int[] arr = { 0, 1, 3, 4, 5 };
         Console.WriteLine(a.MissingNumber1(arr));
         Console.WriteLine(a.MissingNumber2(arr));
         Console.WriteLine(a.MissingNumber3(arr));
         Console.ReadLine();
      }
   }
}

Output

2
2
2