# What are the different ways to find missing numbers in a sorted array without any inbuilt functions using C#?

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));
}
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