Root: First = 1; Last = 5; Next = ; Previous = ;
1: First = 11; Last = 15; Next = 2; Previous = ;
2: First = 21; Last = 25; Next = 3; Previous = 1;
...
554: First = ; Last = ; Next = 555 ; Previous = 553;
555: First = ; Last = ; Next = ; Previous = 554;