lg2863[usaco06jan]牛的舞会

#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<algorithm>
#define rep(i,x,y) for(register int i=x;i<=y;++i)
#define repd(i,x,y) for(register int i=x;i>=y;--i)
#define ll long long
using namespace std;
const int N=1e5;
struct {
    int v,nxt;
    inline void init(int a,int b){v=a;nxt=b;}
}e[N];
int dfn[N],low[N],in[N],ans,cnt,head[N],n,m,stck[N],indx;
inline void adde(int a,int b){
    e[cnt].init(b,head[a]);
    head[a]=cnt++;
}

void tarjan(int u){
    in[u]=1;
    low[u]=dfn[u]=++cnt;
    stck[++indx]=u;
    for(int i=head[u],v;i!=-1;i=e[i].nxt){
        if(!dfn[v=e[i].v]){tarjan(v); low[u]=min(low[u],low[v]);}
        else if(in[v])low[u]=min(low[u],dfn[v]);
    }
    if(low[u]==dfn[u]){
        int k=0;
        do{
            in[indx]=0;
            --indx;
            k++;
        }while(stck[indx+1]!=u);
        if(k>1)ans++;
    }
}

int main(){
    scanf("%d%d",&n,&m);
    memset(head,-1,sizeof(head));
    rep(i,1,m){
        int x,y;
        scanf("%d%d",&x,&y);
        adde(x,y);
        
    }
    cnt=0;
    rep(i,1,n){
        if(!dfn[i])tarjan(i);
    }
    printf("%dn",ans);
    return 0;
}