【luogu2341】 [HAOI2006]受欢迎的牛 [tarjan 缩点]

#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<cmath>
#include<vector>
#include<stack>
#include<algorithm> 
using namespace std;
const int N=10000+5,M=50000+5;
int n,m,dfn[N],low[N],inst[N],bl[N],idx=0,Bcnt=0;
int sum[N],r[N];
stack<int>s;
vector<int>g[N];
template <class t>void rd(t &x){
    x=0;int w=0;char ch=0;
    while(!isdigit(ch)) w|=ch=='-',ch=getchar();
    while(isdigit(ch)) x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
    x=w?-x:x;
}

int head[N],tot=0;
struct edge{int u,v,nxt;}e[M];
void add(int u,int v){
    e[++tot]=(edge){u,v,head[u]};head[u]=tot;
}

void tarjan(int u){
    dfn[u]=low[u]=++idx;
    inst[u]=1,s.push(u);
    for(int i=head[u],v;i;i=e[i].nxt){
        v=e[i].v;
        if(!dfn[v]) tarjan(v),low[u]=min(low[u],low[v]);
        else if(inst[v]&&dfn[v]<low[u]) low[u]=dfn[v];
    }
    if(dfn[u]==low[u]){
        ++Bcnt;
        int v;
        do{
            v=s.top();s.pop();
            bl[v]=Bcnt;
            ++sum[Bcnt];
            inst[v]=0;
        }while(u!=v);
    }
}

int main(){
    freopen("in.txt","r",stdin);
    rd(n),rd(m);
    for(int i=1,u,v;i<=m;++i){
        rd(u),rd(v);
        add(v,u);
    }
    memset(dfn,0,sizeof(dfn));
    for(int i=1;i<=n;++i)
    if(!dfn[i]) tarjan(i);
    for(int i=1;i<=tot;++i)
    if(bl[e[i].u]!=bl[e[i].v]) ++r[bl[e[i].v]];
    int ans,cnt=0;
    for(int i=Bcnt;i;--i)
    if(!r[i]) ans=sum[i],++cnt;
    if(cnt==1) printf("%d",ans);
    else printf("0");
    return 0;
}