public int outDegreeOf(V vertex) {
   Set<E> res = outgoingEdgesOf(vertex);
   return res.size();
 }
 /**
  * .
  *
  * @return
  */
 public int edgeCount() {
   return vertexEdges.size();
 }
 public int inDegreeOf(V vertex) {
   Set<E> res = incomingEdgesOf(vertex);
   return res.size();
 }