Output Operator Overloading

 ostream&  operator<<(ostream  &os,
                                      const  List  &list)  {
         Node  *c  =  list.head;
         if  (c)  {
                 os  <<  c.number;
                 c  =  c->next;
         }
         for  (;  c;  c  =  c->next)
                 os  <<  '  '  <<  c.number;
         return  os;
 }