```java package ds.impl; public class Digraph { private static final String NEW_LINE = System.getProperty("line.separator"); private final int V; private int E; private LinkedBag<Integer>[] adj; private int[] indegree; public Digraph(int V) { if (V < 0) { throw new IllegalArgumentException(); } this.V = V; this.E = 0; indegree = new int[V]; adj = (LinkedBag<Integer>[]) new LinkedBag[V]; for (int v = 0; v < V; v++) { adj[v] = new LinkedBag<>(); } } public Digraph(Digraph G) { this(G.V()); this.E = G.E(); for (int v = 0; v < V; v++) { this.indegree[v] = G.indegree[v]; } for (int v = 0; v < G.V(); v++) { LinkedStack<Integer> reverse = new LinkedStack<>(); for (int w : G.adj[v]) { reverse.push(w); } for (int w : reverse) { adj[v].add(w); } } } public int V() { return V; } public int E() { return E; } private void validateVertex(int v) { if (v < 0 || v >= V) { throw new IllegalArgumentException(); } } public void addEdge(int v1, int v2) { validateVertex(v1); validateVertex(v2); adj[v1].add(v2); indegree[v2]++; E++; } public Iterable<Integer> adj(int v) { validateVertex(v); return adj[v]; } public int outdegree(int v) { validateVertex(v); return adj[v].size(); } public int indegree(int v) { validateVertex(v); return indegree[v]; } public Digraph reverse() { Digraph result = new Digraph(V); for (int v = 0; v < V; v++) { for (int w : adj[v]) { result.addEdge(w, v); } } return result; } @Override public String toString() { StringBuilder s = new StringBuilder(); s.append("V=").append(V).append(", E=").append(E).append(NEW_LINE); for (int v = 0; v < V; v++) { s.append(v).append(": "); for (int w : adj[v]) { s.append(w).append(" "); } s.append(NEW_LINE); } return s.toString(); } public static void main(String[] args) { Digraph g = new Digraph(4); g.addEdge(1, 2); g.addEdge(2, 3); System.out.println(g); } } ```