code: public void addEdge(DiEdge e) {
          if (edges.add(e)) {
              int s = e.get1();
              int t = e.get2();
              addNode(s);
              addNode(t);
              predecessors.get(t).add(s);
              successors.get(s).add(t);
          }
      }
docs: Add the edge (and the nodes) to the graph if they aren't already there
  
  @param s
  @param t