1  import java.util.ArrayList;

  2  

  3  /**

  4     A tower containing disks in the Towers of Hanoi puzzle.

  5  */

  6  public class Tower

  7  {

  8     private ArrayList<Integer> disks;

  9  

 10     /**

 11        Constructs a tower holding a given number of disks of decreasing size.

 12        @param ndisks the number of disks

 13     */

 14     public Tower(int ndisks)

 15     {

 16        disks = new ArrayList<Integer>();

 17        for (int d = ndisks; d >= 1; d--) { disks.add(d); }

 18     }

 19  

 20     /**

 21        Removes the top disk from this tower.

 22        @return the size of the removed disk

 23     */

 24     public int remove()

 25     {      

 26        return disks.remove(disks.size() - 1);

 27     }

 28  

 29     /**

 30        Adds a disk to this tower.

 31        @param size the size of the disk to add

 32     */

 33     public void add(int size)

 34     {

 35        if (disks.size() > 0 && disks.get(disks.size() - 1) < size)

 36        {

 37           throw new IllegalStateException("Disk is too large");

 38        }

 39        disks.add(size);

 40     }

 41  

 42     public String toString() { return disks.toString(); }   

 43  }

 44