Get the number(s) missing from an array. C#. XOR. : Working example : https://dotnetfiddle.net/PKpGEm

getNumber_s_Missing_From_array.cs

using System;
					
public class Program
{
	
	public static int GetMissingNumberFromArray(int[] arr)
	{
		Console.WriteLine("=======================");
		int totalXor = 0;
		int totalArrXor = 0;
		
		for(int i = 1; i <= arr.Length + 1; i++){
			totalXor ^= i;
		}
		Console.WriteLine("totalXor : {0}", totalXor);
		
		
		foreach(int i in arr) totalArrXor ^= i;
		Console.WriteLine("totalArrXor : {0}", totalArrXor);
		
		return totalXor ^ totalArrXor;

	}
	
	public static int[] GetMissingTwoNumbers(int[] arr){
		
		int size = arr.Length + 2;
		Console.WriteLine("size : {0}", size);
		
		long totalSum = size*(size+1)/2;
		Console.WriteLine("totalSum : {0}", totalSum);
		
		long arrSum = 0;
		foreach(int i in arr) arrSum += i;
		Console.WriteLine("arrSum : {0}", arrSum);
		
		int pivot = (int)((totalSum - arrSum) / 2);
		Console.WriteLine("pivot : {0}", pivot);
		
		
		int totalXorRight = 0;
		int totalXorArrayRight = 0;
		int totalXorLeft = 0;
		int totalXorArrayLeft = 0;
		
		//find first array
		for(int i = 1; i <= pivot; i++) totalXorLeft ^= i;
		Console.WriteLine("totalXorLeft : {0}", totalXorLeft);
		
		//find second
		for(int i = pivot +1; i <= size; i++) totalXorRight ^= i;
		Console.WriteLine("totalXorRight : {0}", totalXorRight);
		
		foreach(int i in arr) {
			if(i <= pivot){
				totalXorArrayLeft ^= i;
			} else {
				totalXorArrayRight ^= i;
			}
		}
		
		Console.WriteLine("totalXorArrayLeft : {0}", totalXorArrayLeft);
		
		Console.WriteLine("totalXorRight : {0}", totalXorArrayRight);
		
		
		return new int[] {(totalXorLeft ^ totalXorArrayLeft), (totalXorRight ^ totalXorArrayRight)};
	}
	
	
	
	
	public static void Main()
	{
		int[] testArrayMissing2 = {1,2,5,6};
		
		int[] testArrayMissing1 = {1,2,4,5,6};
		
		int[] missingTwoNumbers = GetMissingTwoNumbers(testArrayMissing2);
		
		int missingOneNumber = GetMissingNumberFromArray(testArrayMissing1);
		
		
		Console.WriteLine("=======================");
		
		
		foreach(int i in missingTwoNumbers) Console.WriteLine("Number Missing : {0}", i);
		
		
		Console.WriteLine("--------------------");
		
		
		Console.WriteLine("Number Missing single : {0}", missingOneNumber);
		
		
	}
}