Popular Cows 强连通(kosaraju)

#include <cstdio>
#include <vector>
#include <cstring>
#include <algorithm>
using namespace std;

int n,k;
const int maxn=10010;
vector<int> g[maxn];
int V;//顶点
vector<int> rg[maxn],vs;
bool used[maxn];
int belong[maxn];

void add_edge(int from,int to){
    g[from].push_back(to);
    rg[to].push_back(from);
}

void dfs(int v){
    used[v]=1;
    for(int i=0;i<g[v].size();i++){
        if(!used[g[v][i]]) dfs(g[v][i]);
    }
    vs.push_back(v);
}

void rdfs(int v,int k){
    used[v]=1;
    belong[v]=k;
    for(int i=0;i<rg[v].size();i++)
        if(!used[rg[v][i]]) rdfs(rg[v][i],k);
}
int scc(){
    k=0;
    memset(used,0,sizeof(used));
    vs.clear();
    for(int v=1;v<=n;v++){
        if(!used[v]) dfs(v);
    }
    memset(used,0,sizeof(used));
    for(int i=vs.size()-1;i>=0;i--)
        if(!used[vs[i]])    rdfs(vs[i],k++);
    return k;
}
int main(){
    int m,a,b;
    //freopen("data.in","r",stdin);
    while(~scanf("%d%d",&n,&m)){
        for(int i=0;i<m;i++){
            scanf("%d%d",&a,&b);
            add_edge(a,b);
        }
        int nn= scc(),u=0,num=0;
        //统计备选解的个数
        for(int i=1;i<=n;i++){
            if(belong[i]==nn-1){
                u=i;num++;
            }
        }
        //检查是否从所有点可达
        memset(used,0,sizeof(used));
        rdfs(u,0);
        for(int v=1;v<=n;v++){
            if(!used[v]){
                num=0;break;
            }
        }
        printf("%d\n",num);
    }
}