@Override
    public int compare(Vertex<Integer> o1, Vertex<Integer> o2) {
        if(o1.getDegree() <= o2.getDegree()){
            return 1;
        }else{
            return -1;
        }
    }
http://www.geeksforgeeks.org/graph-coloring-set-2-greedy-algorithm/
src/com/interview/graph/GraphColoring.java