tarjan 축소. - 인기 소. - 노트.

#include///     hzwer ，%%%hzwer 
#include 
#include 
#include
#include
#define MAXN 10005
#define MAXM 50005
using namespace std;
struct data{
	int to,next;
}e[MAXM],d[MAXM];
int head[MAXN];
int n,m,cnt,top;
int dfn[MAXN],low[MAXN],q[MAXN];
int scc,h[MAXN],belong[MAXN],hav[MAXN];
bool vis[MAXN],inq[MAXN];
int ans;
void dfs(int a){
	int now;
	vis[a]=inq[a]=1;
	low[a]=dfn[a]=++cnt;
	q[++top]=a;
	int c=head[a];
	while(c){///        
		if(!vis[e[c].to ]){
			dfs(e[c].to );
			low[a]=min(low[a],low[e[c].to ]);
		}
		else if(inq[e[c].to ]) low[a]=min(low[a],dfn[e[c].to ]);
		c=e[c].next ;
	}
    if(low[a]==dfn[a]){////  a  SCC   
    	scc++;
    	while(now!=a){////   SCC   
    		now=q[top--];
    		inq[now]=0;
    		belong[now]=scc;
    		++hav[scc];
		}
	}
}
void rebuild(){///SCC   ，    
	cnt=0;     
	for(int i=1;i<=n;i++){
		int c=head[i];
		while(c){//// i        
			if(belong[i]!=belong[e[c].to]){
			 d[++cnt].to =belong[e[c].to ];
			 d[cnt].next =h[belong[i]];
			 h[belong[i]]=cnt;
		    }
		    c=e[c].next ;
        }
    }
} 
void tarjan(){
	for(int i=1;i<=n;i++) if(!vis[i]) dfs(i);
	rebuild();
}
void work(){
	for(int i=1;i<=scc;i++)
	  if(!h[i]){/////           
	  	if(ans){
		 ans=0;return; 
	    }
		else ans=hav[i];
	  }
	
}
int main(){
	scanf("%d%d",&n,&m);
	int a,b;
	for(int i=1;i<=m;i++){
		scanf("%d%d",&a,&b);
		e[++cnt].to =b;
		e[cnt].next =head[a];
		head[a]=cnt;
	}
	tarjan();
	work();
	printf("%d",ans);
	return 0;
}