有向图的基本算法

　　有向图的基本定义：由一组顶点和一组有向边组成，每条有向边连接着有序的一对顶点。

import java.util.InputMismatchException;
import java.util.NoSuchElementException;

public class Digraph {
    private final int V;           // number of vertices in this digraph
    private int E;                 // number of edges in this digraph
    private Bag<Integer>[] adj;    // adj[v] = adjacency list for vertex v
    private int[] indegree;        // indegree[v] = indegree of vertex v
    
    public Digraph(int V) {
        if (V < 0) throw new IllegalArgumentException("Number of vertices in a Digraph must be nonnegative");
        this.V = V;
        this.E = 0;
        indegree = new int[V];
        adj = (Bag<Integer>[]) new Bag[V];
        for (int v = 0; v < V; v++) {
            adj[v] = new Bag<Integer>();
        }
    }
   
    public int V() {
        return V;
    }

    public int E() {
        return E;
    }

    public void addEdge(int v, int w) {
        validateVertex(v);
        validateVertex(w);
        adj[v].add(w);
        indegree[w]++;
        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 reverse = new Digraph(V);
        for (int v = 0; v < V; v++) {
            for (int w : adj(v)) {
                reverse.addEdge(w, v);
            }
        }
        return reverse;
    } 
}

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