Popular cattle (Tarjan shrink point template)

#include<cstdio>
#include<cstring>
#include<stack>
using namespace std;

int n,m,head[10005],vis[10005],dfn[10005],
low[10005],color[10005],num[10005],out[10005];

int sum,cnt,tot,jia,ans;

stack<int> s;

struct edge{
    int v,next;
}e[50005];

inline void add(int u,int v){
    e[++cnt].v=v;
    e[cnt].next=head[u];
    head[u]=cnt;
}

inline void tarjan(int u){
    vis[u]=1;
    s.push(u);
    dfn[u]=low[u]=++tot;
    for(int i=head[u];i!=-1;i=e[i].next){
        int v=e[i].v;
        if(!dfn[v]){
            tarjan(v);
            low[u]=min(low[v],low[u]);
        }
        else if(vis[v])low[u]=min(low[v],low[u]);
    }
    if(dfn[u]==low[u]){
        color[u]=++sum;
        vis[u]=0;
        while(1){
            int x=s.top();
            s.pop();
            vis[x]=0;
            color[x]=sum;
            num[sum]++;
            if(x==u)break;
        }
    }
}

inline void shrink(){
    for(int u=1;u<=n;u++){
        for(int i=head[u];i!=-1;i=e[i].next){
            int v=e[i].v;
            if(color[v]!=color[u])out[color[u]]++;
        }
    }
}

int main(){
    memset(head,-1,sizeof(head));
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++){
        int u,v;
        scanf("%d%d",&u,&v);
        add(u,v);
    }
    for(int i=1;i<=n;i++){
        if(!dfn[i])tarjan(i);
    }
    shrink();
    for(int i=1;i<=sum;i++){
        if(out[i]==0){
            jia++;
            ans=num[i];
        }
    }
    if(jia==1)printf("%d\n",ans);
    else printf("%d\n",0);
}