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);

    }

}