【强连通分量】bzoj 1051 受欢迎的牛

# include<cstdio>
# include<cstring>
# include<stack>
# include<algorithm>
# include<iostream>
using namespace std;
const int N=10000+10;
const int M=50000+10;
stack<int>S;
int n,m,ecnt,scc_cnt,dfs_clock,tot,out_degree,cur;
int fist[N],next[M],v[M],pre[N],low[N],scc_no[N],size[N],out[N];
void built(int a,int b){
    ++ecnt;
    v[ecnt]=b;
    next[ecnt]=fist[a];
    fist[a]=ecnt;
}
int check(int a,int b){
    for(int e=fist[a];e!=-1;e=next[e])
    if(v[e]==b)return 0;
    return 1;
}
void init(){
    int a,b;
    memset(fist,-1,sizeof(fist));
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++){
        scanf("%d%d",&a,&b);
    //    if(check(a,b))
        built(a,b);
    }
}
int dfs(int u){
    int lowu=pre[u]=++dfs_clock;
    S.push(u);
    for(int e=fist[u];e!=-1;e=next[e])
    if(!pre[v[e]])
    lowu=min(lowu,dfs(v[e]));
    else if(!scc_no[v[e]])
    lowu=min(lowu,pre[v[e]]);
    low[u]=lowu;
    if(low[u]==pre[u]){
        scc_cnt++;
        for(;;){
            int x=S.top();S.pop();
            scc_no[x]=scc_cnt;
            if(x==u)break;
        }
    }
    return low[u];
}
void find_scc(){
    memset(pre,0,sizeof(pre));
    memset(low,0,sizeof(low));
    for(int i=1;i<=n;i++)
    if(!pre[i])dfs(i);
}
void work(){
    for(int i=1;i<=n;i++)for(int e=fist[i];e!=-1;e=next[e])
    if(scc_no[i]!=scc_no[v[e]])out[scc_no[i]]++;
    for(int i=1;i<=scc_cnt;i++)if(out[i]==0)out_degree=i;
    for(int i=1;i<=n;i++)if(scc_no[i]==out_degree)tot++;
    printf("%d",tot);
}
int main(){
    init();
    find_scc();
    work();
    return 0;
}