Getting a different number - allow you to change array

GettingADifferentNumber_ChangeArray.cs

using System;

class Solution
{
    public static int GetDifferentNumber(int[] arr) // [1, 3, 2, 100]
    {
        if(arr == null || arr.Length == 0) //false
        {
          return 0; 
        }
      
        scanArraySwap(arr); 
      
        return findFirstMissingNumber(arr); 
    }
  
    private static void scanArraySwap(int[] arr) 
    {
      int length = arr.Length; // 4
      
      int index = 0; 
      while(index < length)  /// [100, 1, 2, 3]
      {
        var number = arr[index]; // 1
        var isLessThanSize = number < length; // true, false 
        if(isLessThanSize)
        {
          var twoNumbers = index != arr[index]; 
          if(twoNumbers)   // bug in writing 
          {
            swap(arr, index, arr[index]); // 0, 1; 0, 3
          }
          else 
          {
            index++; 
          }
        }
        else
        {
          index ++; // 1
        }
      }
    }
  
  private static int findFirstMissingNumber(int[] arr)
  {
      int length = arr.Length; 
      for(int i = 0; i < length; i++)
      {
        if(arr[i] != i)
        {
          return i; 
        }        
      }
    
      return length; 
  }
  
   private static void swap(int[] arr, int pos1, int pos2)
   {
     int tmp = arr[pos1]; 
     arr[pos1] = arr[pos2]; 
     arr[pos2] = tmp; 
   }
  
  
  
    static void Main(string[] args)
    {
       Console.WriteLine(GetDifferentNumber(new int[]{0, 1, 2, 3})) ; 
    }
}
// [0, 1, 2, 3]  - 
// 4 
// array size 4
// int[4] - 0 
// index = 0, 
// found[index] = 1, found[2] = 1, > 4, 
// scan minimum index with 0 -> size = 4
// O(n) space, time complexity O(n)
// 
// [1, 3, 2, 100] -> 0 
// size of 4
// [3, 1, 2, 100]
// [100, 1, 2, 3]
// 100 -> outside 100 - 
// 1 -> 
// time complexity once - O(n)