Note that the STL priority_queue class is missing the decrease_key operation required by Dijkstra's algorithm, so you couldn't use it even if it were permitted. Which it isn't. Good thing you happen to have some old priority queue implementation lying around from assignment 3 that you can adapt. Doing a linear search instead of a log-time search for the next planet will result in suboptimal performance of the program and consequently a suboptimal grade.