bool mapClass::IsPath(int OriginCity, int DestinationCity)
// -----------------------------------------------------
// Determines whether a sequence of flights between two
// cities exists. Nonrecursive stack version.
// Precondition: OriginCity and DestinationCity are the city
// numbers of the origin and destination cities, 
// respectively.
// Postcondition: Returns true if a sequence of flights
// exists from OriginCity to DestinationCity, otherwise 
// returns false. Cities visited during the search are 
// marked as visited in the flight map.
// Implementation notes: Uses a stack S for the city 
// numbers of a potential path. Calls UnvisitAll, 
// MarkVisited, and GetNextCity.
// -----------------------------------------------------
{
   stackClass S;
   int        TopCity, NextCity;
   bool       Success;

   UnvisitAll();  // clear marks on all cities

   // push origin city onto stack, mark it visited
   S.Push(OriginCity, Success);
   MarkVisited(OriginCity);

   S.GetStackTop(TopCity, Success);
   while (!S.StackIsEmpty() && (TopCity != DestinationCity))
   {  // loop invariant: stack S contains a directed path 
      // from the origin city at the bottom of S to the 
      // city at the top of S

      // find an unvisited city adjacent to the city on the
      // top of the stack
      GetNextCity(TopCity, NextCity, Success);

      if (!Success)
         S.Pop(Success);  // no city found; backtrack

      else                // visit city
      {  S.Push(NextCity, Success);
         MarkVisited(NextCity);
      }  // end if

      S.GetStackTop(TopCity, Success);
   }  // end while

   if (S.StackIsEmpty())
      return false;  // no path exists
   else
      return true;   // path exists
}  // end IsPath