Proof of detecting the start of cycle in linked list [duplicate]

Answer 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;
}