Find loops in a linked list.

There are several algorithms floating on the net to detect a cycle/loop in a linked list. The following code detects cycle in a linked list in time O(N) and space O(1).

/*Algorithm : P2 is moving through the list twice as fast as P1.
  If the list is circular, it will eventually get around P1 and meet */

  public boolean hasCycle()
  {
      DoubleNode p1,p2;
      p1=p2=firstNode;    //Start with the first loop
   
      try
     {
        while (p2 != null)     //If p2 reaches end of linked list, no cycle exists
        {
            p1=p1.next;   //Move to next
            p2=p2.next.next; //Move to 2 steps next
            if(p1==p2)
               return true;     //p1 and p2 met, so this means that there is a cycle
        }
      }
      catch(NullPointerException npe)
      {
          //This means that p2 could not move forward
          return false;
      }

      return false;
  }