int totalCount = arr.Count;

 List<int> a = new List<int>(totalCount);
 for(int i =0;i< 100;i++)
 {
     a.Add(0);
 }
 int count = 0;
 for (int i = 0; i < arr.Count; i++)
 {
     if (a[arr[i]] > 0)
     {
         a[arr[i]] =++a[arr[i]];
     }
     else
     {
         a[arr[i]] = ++count;
     }
     count = 0;
 }