class ListNode {
  double    item;
  ListNode  next;
}

class DebugMe {

  public static void main( String[] argv ) {

    int      N = 3;
    ListNode geomSeries = createGeomSeries( 1/2, N );
    double   sum = listSum( geomSeries );

    System.out.println( "for N == " + N + ",\n" );
    System.out.println( "    N  | 1 |^i" );
    System.out.println( "   sum |---| = " + sum );
    System.out.println( "   i=0 | 2 |\n" );
  }

  /**
   *   pre :  N >= 0
   *
   *  post :  returns a linked-list where each node contains
   *          one of the terms in the geometric sum:
   *
   *               N
   *              sum (r^i)
   *              i=0
   */
  public static ListNode createGeomSeries( double r, int N ) {
    ListNode newTerm = new ListNode();
    if( N == 0 ) {
      newTerm.item = 1.0;
      newTerm.next = null;
    }
    else {
      newTerm.next = createGeomSeries( r, N - 1 );
      newTerm.item = newTerm.next.item * r;
    }

    return newTerm;
  }


  /**
   *  post :  computes the sum of terms in a list
   */
  public static double listSum( ListNode l ) {
    if( l == null )
      return 0.0;

    return l.item + listSum(l.next);
  }
}

// eof