CCF 201509-4 高速公路[更正tarjan模板]

#include<stdio.h>
#include<vector>
#include<stack>
using namespace std;
const int N=1e4+5;
int n,m,sd,ans,pd,low[N],dfn[N],sum[N];
bool mark[N];
vector<int>g[N];stack<int>s;
void tarjan(int u){
    low[u]=dfn[u]=++pd;
    mark[u]=1;
    s.push(u);
    for(int &w:g[u]){
        if(!dfn[w]){
            tarjan(w);
            low[u]=min(low[u],low[w]);
        }
        else if(mark[w]){
            low[u]=min(low[u],dfn[w]);
        }
    }
    if(low[u]==dfn[u]){
        sd++;
        for(int x;;){
            x=s.top();s.pop();
            sum[sd]++;
            mark[x]=0;
            if(x==u) break;
        }
    }
}
int main(){
    scanf("%d%d",&n,&m);
    for(int i=1,x,y;i<=m;i++) 
        scanf("%d%d",&x,&y),g[x].emplace_back(y);
    for(int i=1;i<=n;i++) if(!dfn[i]) tarjan(i);
    for(int i=1;i<=sd;i++) ans+=(sum[i]-1)*sum[i]/2;
    printf("%d\n",ans);
    return 0;
}