(Get bipartite sets) Add a new method in AbstractGraph with the following header to return two bipartite sets if the graph is bipartite:

(Get bipartite sets) Add a new method in AbstractGraph with the following header to return two bipartite sets if the graph is bipartite:

public List<List<Integer>> getBipartite();

The method returns a List that contains two sublists, each of which contains a set of vertices. If the graph is not bipartite, the method returns null.