POJ 2186 Popular Cows

http://poj.org/problem?id=2186

留个缩点模板

#include <iostream>
#include <vector>
#include <stack>
#include <cstring>
using namespace std;
const int maxn=10000+10;
int Index=0,cnt=0,N,M,DFN[maxn],Low[maxn],belong[maxn],d[maxn];
bool InS[maxn];

vector<int> G[maxn];
stack <int> S;

int Tarjan(int u)
    {
        DFN[u]=Low[u]=++Index;
        S.push(u);InS[u]=1;
        vector<int>::iterator ite;
        for(ite=G[u].begin();ite!=G[u].end();ite++){
            if(!DFN[*ite]){
                Tarjan(*ite);
                Low[u]=min(Low[u],Low[*ite]);
                }
            else{if(InS[*ite])Low[u]=min(Low[u],DFN[*ite]);}
        }
        if(Low[u]==DFN[u]){
            cnt++;int i;//cout<<"cnt="<<cnt<<endl;
            do{
                i=S.top();S.pop();
                belong[i]=cnt;
                InS[i]=0;
                //cout<<"  "<<i<<endl;
            }while(i!=u);
        }
        //cout<<"========="<<endl;
    return 0;
    }

int main()
{
    ios::sync_with_stdio(0);
    cin>>N>>M;
    memset(DFN,0,sizeof(DFN));
    memset(InS,0,sizeof(InS));
    for(int i=0;i<M;i++){
        int A,B;cin>>A>>B;
        G[A].push_back(B);    
    }
    for(int i=0;i<N;i++)
        if(!DFN[i+1])Tarjan(i+1);
    vector<int>::iterator ite; 
    for(int i=0;i<N;i++)
        for(ite=G[i+1].begin();ite!=G[i+1].end();ite++)
            if(belong[i+1]!=belong[*ite])
                d[belong[i+1]]++;
    int ID=-1,ans=1;
    for(int i=0;(i<N)&&(ans>=0);i++)
        if(!d[belong[i+1]]){
            if(ID<0) ID=belong[i+1];
                else {
                    if(belong[i+1]!=ID) ans=-1;
                                   else ans++;
                }
        }
    cout<<max(ans,0)<<endl;
    return 0;
}