code: public void addEdge(int from, int to) {
          Set<Integer> outEdges = edges.get(from);
          if (outEdges == null) {
              outEdges = new TreeSet<Integer>();
              edges.put(from, outEdges);
          }
          outEdges.add(to);
      }
docs: Add an edge from node <code>from</code> to node <code>to</code>.
  
  @param from
  @param to