挑战程序竞赛系列（75）：4.3强连通分量分解（2）

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.StringTokenizer;

public class Main{

    String INPUT = "./data/judge/201709/P3180.txt";

    public static void main(String[] args) throws IOException {
        new Main().run();
    }

    static final int MAX_N = 10000 + 16;
    List<Edge>[] g, rg;
    List<Integer> po;
    boolean[] used;
    int[] cmp;
    int V;

    class Edge{
        int from;
        int to;
        Edge(int from, int to){
            this.from = from;
            this.to   = to;
        }
    }

    void init(int V) {
        this.V = V;
        g    = new ArrayList[MAX_N];
        rg   = new ArrayList[MAX_N];
        po   = new ArrayList<Integer>();
        used = new boolean[MAX_N];
        cmp  = new int[MAX_N];
        Arrays.fill(used, false);
        for (int i = 0; i < V; ++i) g[i]  = new ArrayList<Edge>();
        for (int i = 0; i < V; ++i) rg[i] = new ArrayList<Edge>();
    }

    void add(int from, int to) {
        g[from].add(new Edge(from, to));
        rg[to].add(new Edge(to, from));
    }

    void dfs(int v) {
        used[v] = true;
        for (Edge e : g[v]) {
            if (!used[e.to]) dfs(e.to);
        }
        po.add(v);
    }

    void rdfs(int v, int k) {
        used[v] = true;
        cmp[v]  = k;
        for (Edge e : rg[v]) {
            if (!used[e.to]) rdfs(e.to, k);
        }
    }

    int kosarajuSCC() {
        for (int i = 0; i < V; ++i) {
            if (!used[i]) dfs(i);
        }
        Arrays.fill(used, false);
        int k = 0;
        for (int i = po.size() - 1; i >= 0; --i) {
            if (!used[po.get(i)]) rdfs(po.get(i), k++);
        }
        return k;
    }

    void read() {
        int N = ni();
        int M = ni();
        init(N);
        for (int i = 0; i < M; ++i) {
            int from = ni();
            int to   = ni();
            from --;
            to   --;
            add(from, to);
        }
        int scc = kosarajuSCC();
        int[] count = new int[scc + 1];
        for (int i = 0; i < V; ++i) {
            count[cmp[i]]++;
        }
        int ans = 0;
        for (int i = 0; i <= scc; ++i) {
            if (count[i] >= 2) ans ++;
        }
        out.println(ans);
    }

    FastScanner in;
    PrintWriter out;

    void run() throws IOException {
        boolean oj;
        try {
            oj = ! System.getProperty("user.dir").equals("F:\\java_workspace\\leetcode");
        } catch (Exception e) {
            oj = System.getProperty("ONLINE_JUDGE") != null;
        }

        InputStream is = oj ? System.in : new FileInputStream(new File(INPUT));
        in = new FastScanner(is);
        out = new PrintWriter(System.out);
        long s = System.currentTimeMillis();
        read();
        out.flush();
        if (!oj){
            System.out.println("[" + (System.currentTimeMillis() - s) + "ms]");
        }
    }

    public boolean more(){
        return in.hasNext();
    }

    public int ni(){
        return in.nextInt();
    }

    public long nl(){
        return in.nextLong();
    }

    public double nd(){
        return in.nextDouble();
    }

    public String ns(){
        return in.nextString();
    }

    public char nc(){
        return in.nextChar();
    }

    class FastScanner {
        BufferedReader br;
        StringTokenizer st;
        boolean hasNext;

        public FastScanner(InputStream is) throws IOException {
            br = new BufferedReader(new InputStreamReader(is));
            hasNext = true;
        }

        public String nextToken() {
            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    hasNext = false;
                    return "##";
                }
            }
            return st.nextToken();
        }

        String next = null;
        public boolean hasNext(){
            next = nextToken();
            return hasNext;
        }

        public int nextInt() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Integer.parseInt(more);
        }

        public long nextLong() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Long.parseLong(more);
        }

        public double nextDouble() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Double.parseDouble(more);
        }

        public String nextString(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more;
        }

        public char nextChar(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more.charAt(0);
        }
    }
}