bzoj1051: [HAOI2006]受欢迎的牛

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<stack>
using namespace std;
#define rep(i,n) for(int i=1;i<=n;i++)
#define clr(x,c) memset(x,c,sizeof(x))
#define REP(i,s,t) for(int i=s;i<=t;i++)
#define op() clr(head,0);pt=edges;
#define qwq(x) for(edge *o=head[x];o;o=o->next)
int read(){
	int x=0;char c=getchar();bool f=true;
	while(!isdigit(c)){
		if(c==‘-‘) f=false;c=getchar();
	}
	while(isdigit(c)) x=x*10+c-‘0‘,c=getchar();
	return f?x:-x;
} 
const int nmax=10005;
const int maxn=50005;
const int inf=0x7f7f7f7f;
struct edge{
	int to;edge *next;
};
edge edges[maxn],*pt,*head[nmax];
int pre[nmax],sccno[nmax],out[nmax],scc_cnt,dfs_clock;
stack<int>s;
void adde(int u,int v){
	pt->to=v;pt->next=head[u];head[u]=pt++;
}
int dfs(int x){
	int lowu=pre[x]=++dfs_clock;s.push(x);
	qwq(x) {
		int to=o->to;
		if(!pre[to]) lowu=min(lowu,dfs(to));
		else if(!sccno[to]) lowu=min(lowu,pre[to]);
	}
	if(lowu==pre[x]){
		scc_cnt++;
		while(1){
			int X=s.top();s.pop();
			sccno[X]=scc_cnt;
			if(x==X) break;
		}
	}
	return lowu;
}
void init(){
	op();clr(out,false);clr(pre,0);clr(sccno,0);dfs_clock=scc_cnt=0;
}
int main(){
	init();
	int n=read(),m=read(),u,v,ans=0,cur,cnt=0;
	rep(i,m) u=read(),v=read(),adde(u,v);
	rep(i,n) if(!pre[i]) dfs(i);
	
	rep(i,n) qwq(i) if(sccno[i]!=sccno[o->to]) out[sccno[i]]=true;
	rep(i,scc_cnt) if(!out[i])  cnt++,cur=i;
	if(cnt==1) rep(i,n) if(sccno[i]==cur) ans++;
	printf("%d\n",ans);
}