bzoj1051: [HAOI2006]受欢迎的牛

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int M=50007;
int read(){
    int ans=0,f=1,c=getchar();
    while(c<‘0‘||c>‘9‘){if(c==‘-‘) f=-1; c=getchar();}
    while(c>=‘0‘&&c<=‘9‘){ans=ans*10+(c-‘0‘); c=getchar();}
    return ans*f;
}
int n,m,sum,first[M];
int color[M],out[M],s[M],num,ans,last;
int dfn[M],low[M],cnt,st[M],top,book[M];
struct node{int to,next;}e[M];
void insert(int a,int b){sum++; e[sum].to=b; e[sum].next=first[a]; first[a]=.sum;}
void dfs(int x){
    dfn[x]=low[x]=++cnt;
    book[x]=1; 
    st[++top]=x;
    for(int i=first[x];i;i=e[i].next){
        int now=e[i].to;
        if(!dfn[now]){
            dfs(now);
            low[x]=min(low[x],low[now]);
        }
        if(book[now]) low[x]=min(low[x],dfn[now]);
    }
    if(low[x]==dfn[x]){
        s[++num]=1;
        book[x]=0; color[x]=num;
        while(st[top]!=x){
            int now=st[top--];
            book[now]=0;
            color[now]=num;
            s[num]++;
        }
        top--;
    }
}
int main()
{
    int x,y;
    n=read(); m=read();
    for(int i=1;i<=m;i++) x=read(),y=read(),insert(x,y);
    for(int i=1;i<=n;i++) if(!dfn[i]) dfs(i);
    for(int i=1;i<=n;i++)
        for(int j=first[i];j;j=e[j].next)
            if(color[e[j].to]!=color[i]){
                out[color[i]]=1;
                break;
            }
    for(int i=1;i<=num;i++) if(!out[i]){
        if(last){
            printf("0\n");
            return 0;
        }
        last=i;
    }
    printf("%d\n",s[last]);
    return 0;
}