[HAOI2006]受欢迎的牛（缩点）

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#include<cstdio>
#include<iostream>
using namespace std;
struct node{
int to,next;
}e[50001],d[50001];
int head[10001];
int n,m,cnt,top;
int dfn[10001],low[10001],q[10001];
int scc,h[10001],belong[10001],hav[10001];
bool vis[10001],inq[10001];
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])
{
scc++;
while(now!=a)
{
now=q[top--];
inq[now]=0;
belong[now]=scc;
++hav[scc];
}
}
}
void tarjan()
{
for(int i=1;i<=n;i++)
if(!vis[i])dfs(i);
cnt=0;
for(int i=1;i<=n;i++)
{
int c=head[i];
while(c)
{
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;
}
}
}
int main()
{
scanf("%d%d",&n,&m);
for(int i=1;i<=m;i++)
{
int x,y;
scanf("%d%d",&x,&y);
e[i].to=y;
e[i].next=head[x];
head[x]=i;
}
tarjan();
for(int i=1;i<=scc;i++)
if(!h[i])
{
if(ans)
{
ans=0;
break;
}
else ans=hav[i];
}
printf("%dn",ans);
return 0;
}