缩点

//kosaraju
#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<algorithm>
#include<ctime>
using namespace std;
const int N=10005;
int n,m,cnt,head[N],rhead[N],t,rt,q[N],top,ans,rcnt,Cnt[N],sum[N],number;
bool vis[N],rvis[N];
struct Node{
    int u,v,next;
}edge[N];
struct rNode{
    int u,v,next;
}redge[N];
void push(int u,int v){
    ++cnt;
    edge[cnt].u=u;
    edge[cnt].v=v;
    edge[cnt].next=head[u];
    head[u]=cnt;
}
void rpush(int u,int v){
    ++rcnt;
    redge[rcnt].u=u;
    redge[rcnt].v=v;
    redge[rcnt].next=rhead[u];
    rhead[u]=rcnt;
}
void kosaraju(int u){
    vis[u]=true;
    for(int i=head[u];i!=-1;i=edge[i].next){
        int v=edge[i].v;
        if(!vis[v]){
            kosaraju(v);
        }
    }
    q[++top]=u;
}
void rkosaraju(int u){
    vis[u]=true;
    Cnt[u]=t;
    sum[t]++;
    for(int i=rhead[u];i!=-1;i=redge[i].next){
        int v=redge[i].v;
        if(!vis[v]){
            rkosaraju(v);
        }
        else if(Cnt[u]!=Cnt[v]){
            rvis[Cnt[v]]=false;
        }
    }
}
int main(){
    memset(head,-1,sizeof(head));
    memset(rhead,-1,sizeof(rhead));
    memset(vis,false,sizeof(vis));
    int u,v;
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++){
        scanf("%d%d",&u,&v);
        push(u,v);
        rpush(v,u);
    }
    for(int i=1;i<=n;i++){
        if(!vis[i]){
            kosaraju(i);
        }
    }
    memset(vis,false,sizeof(vis));
    memset(rvis,true,sizeof(rvis));
    for(int i=n;i>=1;i--){
        if(!vis[q[i]]){
            ++t;
            rkosaraju(q[i]);
        }
    }
    for(int i=1;i<=t;i++){
        if(rvis[i]){
            number++;
            ans=sum[i];
        }
    }
    if(number!=1){
        printf("0\n");
    }
    else{
        printf("%d",ans);
    }
    return 0;
}
//tarjan
#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<algorithm>
#include<ctime>
using namespace std;
const int N=50005;
int n,m,cnt,head[N],dfn[N],low[N],t,q[N],top,number,num[N],sum,ans,Cnt[N],last[N];
bool vis[N];
struct Node{
    int u,v,next;
}edge[N];
void push(int u,int v){
    ++cnt;
    edge[cnt].u=u;
    edge[cnt].v=v;
    edge[cnt].next=head[u];
    head[u]=cnt;
}
void tarjan(int u){
    dfn[u]=low[u]=++t;
    q[++top]=u;
    vis[u]=true;
    for(int i=head[u];i!=-1;i=edge[i].next){
        int v=edge[i].v;
        if(dfn[v]==-1){
            tarjan(v);
            low[u]=min(low[u],low[v]);
        }
        if(vis[v]){
            low[u]=min(low[u],dfn[v]);
        }
    }
    if(dfn[u]==low[u]){
        sum++;
        for(;q[top+1]!=u&&top>0;){
            Cnt[q[top]]=sum;
            num[sum]++;
            vis[q[top--]]=false;
        }
    }
}
int main(){
    memset(head,-1,sizeof(head));
    memset(dfn,-1,sizeof(dfn));
    int u,v;
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++){
        scanf("%d%d",&u,&v);
        push(u,v);
    }
    for(int i=1;i<=n;i++){
        if(dfn[i]==-1){
            tarjan(i);
        }
    }
    for(int i=1;i<=n;i++){
        for(int j=head[i];j!=-1;j=edge[j].next){
            int v=edge[j].v;
            if(Cnt[i]!=Cnt[v]){
                last[Cnt[i]]++;
            }
        }
    }
    for(int i=1;i<=sum;i++){
        if(!last[i]){
            number++;
            ans=num[i];
        }
    }
    if(number>1){
        printf("0\n");
    }
    else{
        printf("%d\n",ans);
    }
    return 0;
}