Digraph.java

import edu.princeton.cs.algs4.*;

public class Digraph{
    private final int V;
    private int E;
    private Bag<Integer>[] adj;
    private static final String NEWLINE = System.getProperty("line.separator");

    public Digraph(int V){
        this.V = V;
        this.E = 0;
        adj = (Bag<Integer>[]) new Bag[V];
        for(int v=0; v < V; v++){
            adj[v] = new Bag<Integer>();
        }
    }

    public Digraph(In in) {
        this.V = in.readInt();
        adj = (Bag<Integer>[]) new Bag[V];
        for (int v = 0; v < V; v++) {
            adj[v] = new Bag<Integer>();
        }
        int E = in.readInt();
        for (int i = 0; i < E; i++) {
            int v = in.readInt();
            int w = in.readInt();
            addEdge(v, w); 
        }
    }

    public int V(){
        return V;
    }

    public int E(){
        return E;
    }

    public void addEdge(int v, int w){
        adj[v].add(w);
        E++;
    }

    public Iterable<Integer> adj(int v){
        return adj[v];
    }

    public Digraph inverse(){
        Digraph R = new Digraph(V);
        for(int v = 0; v < V; v++){
            for(int w : adj[v]){
                R.addEdge(w, v);
            }
        }
        return R;
    }

    public String toString(){
        StringBuilder s = new StringBuilder();
        s.append(V + " vertices, " + E + " edges " + NEWLINE);
        for(int v=0; v < V; v++){
            s.append(String.format("%d: ", v));
            for(int w : adj(v)){
                s.append(String.format("%d ", w));
            }
            s.append(NEWLINE);
        }
        return s.toString();
    }

    /**
     * Unit tests the {@code Digraph} data type.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        In in = new In(args[0]);
        Digraph G = new Digraph(in);
        StdOut.println(G);
        StdOut.println(G.inverse());
    }
}