public Digraph reverse() {
   Digraph rDigraph = new Digraph(V);
   for (int v = 0; v < V; v++) {
     for (int w : adj[v]) {
       rDigraph.addEdge(w, v);
     }
   }
   return rDigraph;
 }
  public static void main(String[] args) {

    File tFile = new File("algs4-data//tinyDG.txt");

    In in = new In(tFile);
    Digraph g = new Digraph(in);
    System.out.println(g);
    System.out.println(g.reverse());
  }