[BZOJ 1051][HAOI2006]受欢迎的牛：强连通分量

/*
User:Small
Language:C++
Problem No.:1051
*/
#include<bits/stdc++.h>
#define ll long long
#define inf 999999999
using namespace std;
const int M=1e5+5;
int n,m,fir[M],tot,rtot,rfir[M],col[M],ans,dfn,g[M],u;
bool vis[M];
struct edge{
int v,nex;
}e[5*M],re[5*M];
void add(int u,int v){
e[++tot]=(edge){v,fir[u]};
fir[u]=tot;
re[++rtot]=(edge){u,rfir[v]};
rfir[v]=rtot;
}
void dfs(int u){
vis[u]=1;
for(int i=fir[u];i;i=e[i].nex){
int v=e[i].v;
if(vis[v]==0) dfs(v);
}
g[++dfn]=u;
}
void rdfs(int u,int k){
col[u]=k;
vis[u]=1;
for(int i=rfir[u];i;i=re[i].nex){
int v=re[i].v;
if(vis[v]==0) rdfs(v,k);
}
}
int scc(){
int res=0;
memset(vis,0,sizeof(vis));
for(int i=1;i<=n;i++)
if(!vis[i]) dfs(i);
memset(vis,0,sizeof(vis));
for(int i=n;i;i--)
if(!vis[g[i]]) rdfs(g[i],++res);
return res;
}
int main(){
freopen("data.in","r",stdin);//
scanf("%d%d",&n,&m);
while(m--){
int u,v;
scanf("%d%d",&u,&v);
add(u,v);
}
int k=scc();
for(int i=1;i<=n;i++){
if(col[i]==k){
ans++;
u=i;
}
}
memset(vis,0,sizeof(vis));
rdfs(u,0);
for(int i=1;i<=n;i++){
if(!vis[i]){
ans=0;
break;
}
}
printf("%d\n",ans);
return 0;
}