Find first missing number - modify array -

FindFirstMissingNumber_ModifyArray.cs

using System;

class Solution
{
    public static int GetDifferentNumber(int[] arr)
    {
      if(arr == null)
        return 0; 
      
      var length = arr.Length;       
      
      int index = 0; 
      while(index < length)
      {
        var current = arr[index]; // 10 
        var isInRange = current < length; // false
        if(isInRange)
        {
          if(current == index)
          {
            index++; 
          }
          else 
          {
            // swap  index with current two position
            var tmp = arr[index]; 
            arr[index]   = arr[current]; 
            arr[current] = tmp; 
          }
        }
        else 
        {
          index++; 
        }
      }
      
      // Try to find first element with value 0
      for(int i = 0; i < length; i++)
      {
        if(arr[i] != i) // found[0] = 0 
        {
          return i; 
        }
      }
      
      return length; 
    }

    static void Main(string[] args)
    {
    }
}

 /*
 [0, 2, 3, 4]  -> array size is 4
 [2, 3, 0, 4] -> 
 Try to find zero
 [0, 3, 2, 4]
  -     - 
  
  [4, 3, 0, 2] -> 4 >= 4
  [0, 4, 2, 3] -> 
      - 
    */