Kosaraju算法详解

class Kosaraju {
  private Digraph G;
  private Digraph reverseG; //反向图
  private Stack<Integer> reversePost; //逆后续排列保存在这
  private boolean[] marked;
  private int[] id; //第v个点在几个强连通分量中
  private int count; //强连通分量的数量
  public Kosaraju(Digraph G) {
    int temp;
    this.G = G;
    reverseG = G.reverse();
    marked   = new boolean[G.V()];
    id     = new int[G.V()];
    reversePost = new Stack<Integer>();
    
    makeReverPost(); //算出逆后续排列
    
    for (int i = 0; i < marked.length; i++) { //重置标记
      marked[i] = false;
    }
    
    for (int i = 0; i < G.V(); i++) { //算出强连通分量
      temp = reversePost.pop();
      if (!marked[temp]) {
        count++;
        dfs(temp);
      }
    }
  }
  /*
   * 下面两个函数是为了算出 逆后序排列
   */
  private void makeReverPost() {
    for (int i = 0; i < G.V(); i++) { //V()返回的是图G的节点数
      if (!marked[i])
        redfs(i);
    }
  }
  
  private void redfs(int v) {
    marked[v] = true;
    for (Integer w: reverseG.adj(v)) { //adj(v)返回的是v指向的结点的集合
      if (!marked[w])
        redfs(w);
    }
    reversePost.push(v); //在这里把v加入栈,完了到时候再弹出来,弹出来的就是逆后续排列
  }
  /*
   * 标准的深度优先搜索
   */
  private void dfs(int v) {
    marked[v] = true;
    id[v] = count;
    for (Integer w: G.adj(v)) {
      if (!marked[w])
        dfs(w);
    }
  }
  
  public int count() { return count;}
}

package practice;

import java.util.ArrayList;
import java.util.Stack;

public class TestMain {
  public static void main(String[] args) {
    Digraph a = new Digraph(13);
    a.addEdge(0, 1);a.addEdge(0, 5);a.addEdge(2, 3);a.addEdge(2, 0);a.addEdge(3, 2);
    a.addEdge(3, 5);a.addEdge(4, 3);a.addEdge(4, 2);a.addEdge(5, 4);a.addEdge(6, 0);
    a.addEdge(6, 4);a.addEdge(6, 9);a.addEdge(7, 6);a.addEdge(7, 8);a.addEdge(8, 7);
    a.addEdge(8, 9);a.addEdge(9, 10);a.addEdge(9, 11);a.addEdge(10, 12);a.addEdge(11, 4);
    a.addEdge(11, 12);a.addEdge(12, 9);
    
    Kosaraju b = new Kosaraju(a);
    System.out.println(b.count());
  }
}

class Kosaraju {
  private Digraph G;
  private Digraph reverseG; //反向图
  private Stack<Integer> reversePost; //逆后续排列保存在这
  private boolean[] marked;
  private int[] id; //第v个点在几个强连通分量中
  private int count; //强连通分量的数量
  public Kosaraju(Digraph G) {
    int temp;
    this.G = G;
    reverseG = G.reverse();
    marked   = new boolean[G.V()];
    id     = new int[G.V()];
    reversePost = new Stack<Integer>();
    
    makeReverPost(); //算出逆后续排列
    
    for (int i = 0; i < marked.length; i++) { //重置标记
      marked[i] = false;
    }
    
    for (int i = 0; i < G.V(); i++) { //算出强连通分量
      temp = reversePost.pop();
      if (!marked[temp]) {
        count++;
        dfs(temp);
      }
    }
  }
  /*
   * 下面两个函数是为了算出 逆后序排列
   */
  private void makeReverPost() {
    for (int i = 0; i < G.V(); i++) { //V()返回的是图G的节点数
      if (!marked[i])
        redfs(i);
    }
  }
  
  private void redfs(int v) {
    marked[v] = true;
    for (Integer w: reverseG.adj(v)) { //adj(v)返回的是v指向的结点的集合
      if (!marked[w])
        redfs(w);
    }
    reversePost.push(v); //在这里把v加入栈,完了到时候再弹出来,弹出来的就是逆后续排列
  }
  /*
   * 标准的深度优先搜索
   */
  private void dfs(int v) {
    marked[v] = true;
    id[v] = count;
    for (Integer w: G.adj(v)) {
      if (!marked[w])
        dfs(w);
    }
  }
  
  public int count() { return count;}
}
/*
 * 图
 */
class Digraph {
  private ArrayList<Integer>[] node;
  private int v;
  public Digraph(int v) {
    node = (ArrayList<Integer>[]) new ArrayList[v];
    for (int i = 0; i < v; i++)
      node[i] = new ArrayList<Integer>();
    this.v = v;
  }
  
  public void addEdge(int v, int w) { node[v].add(w);}
  
  public Iterable<Integer> adj(int v) { return node[v];}
  
  public Digraph reverse() {
    Digraph result = new Digraph(v);
    for (int i = 0; i < v; i++) {
      for (Integer w : adj(i))
        result.addEdge(w, i);
    }
    return result;
  }
  
  public int V() { return v;}

}