Find first missing number - using extra array to mark found

FindFirstMissingNumber.cs

using System;

class Solution
{
    public static int GetDifferentNumber(int[] arr)
    {
      if(arr == null)
        return 0; 
      
      var length = arr.Length; 
      
      var found = new int[length]; 
      
      for(int i = 0; i < length; i++)
      {
        var current = arr[i]; // 10 
        var isInRange = current < length; // false
        if(isInRange)
        {
          found[current] = 1; 
        }
      }
      
      // Try to find first element with value 0
      for(int i = 0; i < length; i++)
      {
        if(found[i] == 0) // found[0] = 0 
        {
          return i; 
        }
      }
      
      return length; 
    }

    static void Main(string[] args)
    {

    }
}

 /*
keywords: 

unique, nonnegative, integer, an array

Ask: find smallest nonnegative integer not in the array

Constraint: not allow to modify the input arr. 
  
  [0, 1, 2, 3] -> 4
  Array size is 4, 0 - 3, then missing is 4. answer is <= 3
  
  declare an extra array, [1, 1, 1, 1] -> missing is SIZE = , 0 -> size - 1
  
  
  [4, 3, 2, 1, 0]
  
  Extra array:
  [1,1,1,1,1] -> 5 
    
    iterate original array, update extra array
    
    scan the extra array from left to right, find first element with value 0. 
    
    Time complexity: O(n), n is size of the array
    
    */