Getting a different number - cannot change the array

GettingDifferentNumber_CannotChangeArray.cs

using System;

class Solution
{
    public static int GetDifferentNumber(int[] arr) // [0, 1, 2, 3]
    {
      if(arr == null || arr.Length == 0) //false 
      {
        return 0; 
      }
      
      int length = arr.Length; // 4
      var found = scanArraySaveFound(arr); // 
      
      // go over the found array to find first missing one
      for(int i = 0; i < length; i++) 
      {
        if(found[i] == 0)  // found[i], not arr[i] - 
        {
          return i; 
        }
      }
      
      return length; // 4
    }

    private static int[] scanArraySaveFound(int[] arr)
    {
      int length = arr.Length; 
      var found = new int[length]; // 4
      
      for(int i = 0 ; i < length; i++) // [0, 1, 2, 3]
      {
        var number = arr[i]; // 0 
        if(number >= length) // 4
        {
          continue; 
        }
        
        found[number] = 1; // found[0] = 1
      }
      
      return found; 
    }
  
    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)
//