有向無環圖的拓撲排序，首先定義有向圖的存儲數據結構，鄰接鏈表Bag，實現Iterable接口。

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Scanner;

/**
 * <pre>
 * 任務：
 * 描述：鏈表結構
 * 作者：@author huangpeng
 * 時間：@create 2018-09-02 下午1:07
 * 類名: Bag
 * </pre>
 **/

public class Bag<Item> implements Iterable<Item> {

    private int N;
    private Node<Item> first;

    public int getN() {
        return N;
    }

    public Item getFirstItem() {
        return first.item;
    }

    private class Node<Item>{
        private Item item;
        private Node<Item> next;

        public Item getItem() {
            return item;
        }

        public Node<Item> getNext() {
            return next;
        }
    }

    public Bag(){
        first = null;
        N = 0;
    }

    public boolean isEmpty(){
        return first ==null;
    }

    public int size(){
        return N;
    }

    public void add(Item item){
        Node<Item> oldfirst = first;
        first = new Node<Item>();
        first.item = item;
        first.next = oldfirst;
        N++;
    }

    @Override
    public Iterator<Item> iterator() {
        return new ListIterator<Item>(first);
    }

    private class ListIterator<Item> implements Iterator<Item>{

        private Node<Item> current;

        public ListIterator(Node<Item> first){
            current = first;
        }
        @Override
        public boolean hasNext() {
            return current!=null;
        }

        @Override
        public Item next() {
            if(!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next;
            return item;
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        Bag<String> bag = new Bag<String>();
        while(!sc.hasNext("0")){
            bag.add(sc.next());
        }
        System.out.println("size of bag = "+bag.size());
        for (String s:bag){
            System.out.println(s);
        }
    }
}

定義有向圖的數據結構：

import java.io.File;
import java.io.IOException;
import java.util.*;

/**
 * <pre>
 * 任務：
 * 描述：有向無環圖的存儲，拓撲排序
 * 作者：@author huangpeng
 * 時間：@create 2018-09-02 下午1:50
 * 類名: Diagraph
 * </pre>
 **/

public class Diagraph {
    public int V;
    public int E;
    public Bag<Integer>[] adj;
    public int[] indegree;
    public int[] outdegree;
    int[] num ;

    public Diagraph(int V){
        if(V < 0) throw new IllegalArgumentException();
        this.V = V;
        this.E = 0;
        adj = (Bag<Integer>[]) new Bag[V];
        for(int v=0;v<V;v++){
            adj[v] = new Bag<Integer>();
        }
        this.indegree = new int[V];
        this.outdegree = new int[V];
        this.num = new int[V];
    }

    public Diagraph(String inputFile){
        try {
            File file = new File(inputFile);
            if (file.exists()) {
                Scanner sc = new Scanner(file);
                this.V = sc.nextInt();
                this.indegree = new int[V];
                this.outdegree = new int[V];
                this.num = new int[V];
                if (V < 0) throw new IllegalArgumentException("Number of vertices in a Digraph must be nonnegative");
                adj = (Bag<Integer>[]) new Bag[V];
                for (int v = 0; v < V; v++) {
                    adj[v] = new Bag<Integer>();
                }
                int E = sc.nextInt();
                if (E < 0) throw new IllegalArgumentException("Number of edges in a Digraph must be nonnegative");
                for (int i = 0; i < E; i++) {
                    int v = sc.nextInt();
                    int w = sc.nextInt();
                    addEdge(v, w);
                }
                return;
            }
        }catch(IOException e){
            System.out.println("couldn't open file "+inputFile);
        }

    }

    public int V(){
        return V;
    }
    public int E(){
        return E;
    }
    public int[] getIndegree(){
        return indegree;
    }
    public int[] getOutdegree(){
        return outdegree;
    }

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

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

    public Diagraph reverse(){
        Diagraph R = new Diagraph(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 "+"\n");
        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("\n");
        }
        return s.toString();
    }

    public int indegree(int nodeNumber){
        return indegree[nodeNumber];
    }

    public int outDegree(int nodeNumber){
        return outdegree[nodeNumber];
    }

    public void topologySort(){

        Queue<Integer> queue = new LinkedList<Integer>();
        int counter = 0;

        for(int i=0;i<V;i++){
            if(indegree[i]==0){
                queue.offer(i);
            }
        }

        while(!queue.isEmpty()){
            int v = queue.poll();
            num[v] = counter++;
            for(int i=0;i<V;i++){
                if(v!=i&&check(v,i)&&--indegree[i]==0){
                    queue.offer(i);
                }
            }
        }
    }

    public boolean check(int v,int i){
        Iterator iter = adj(v).iterator();
        while(iter.hasNext()){
            int value = (Integer) iter.next();
            if(value == i){
                return true;
            }
        }
        return false;
    }

    public static void main(String[] args) {
        Diagraph G = new Diagraph("/Users/huangpeng/workspace/javaproj/huangpengProj/src/com/algors/sap/maps/tinyDG1.txt");
        System.out.println(G);
        System.out.println("indegree:"+ Arrays.toString(G.getIndegree()));
        System.out.println("outdegree:"+ Arrays.toString(G.getOutdegree()));
        G.topologySort();
        System.out.println("topology sort:"+Arrays.toString(G.num));
    }
}