用tarjan缩点重建图

尝试一下缩点减新图。代码应当是poj2186的，但是poj崩了，也不知代码对不对。。。

#include
#include
#include
#include
using namespace std;
#define mem(arr,a) memset(arr,a,sizeof(arr))
#define N 10000
#define MAXN 50000
vector<int>G[N];//原图
vector<int>rG[N];//重建的图
vector<int>tG[N];//用来保存新图每个节点对应的连通分量里的点
int dfn[N], low[N], vis[N];
int n, m;
int cnt,counts;
int id[N];//连通分量里的点对应新图点的编号
void add(int a, int b){
    G[a].push_back(b);
}
void tarjan(int u){
    stack<int>s;
    dfn[u] = low[u] = ++cnt;
    vis[u] = 1;
    s.push(u);
    for (int i = 0; i < G[u].size(); i++){
        int v = G[u][i];
        if (!dfn[v]){
            tarjan(v);
            if (low[v] < low[u])low[u] = low[v];
        }
        else if (vis[v] && low[u]>dfn[v])low[u] = dfn[v];
    }
    while (!s.empty()){
        int t = s.top();
        s.pop();
        id[t] = counts;
        tG[counts].push_back(t);
        if (t == u)break;
    }
    counts++;
}
void Tarjan(){
    for (int i = 1; i <= n; i++){
        if (!dfn[i])tarjan(i);
    }
}
int main(){
    cin >> n >> m;
    while (m--){
        int a, b; cin >> a >> b; add(a, b);
    }
    Tarjan();
    mem(vis, 0);
    for (int i = 0; i < counts; i++){
        vis[i] = 0;
        for (int j = 0; j < tG[i].size(); j++){
            int u = tG[i][j];
            for (int k = 0; k < G[u].size(); k++){
                if (!vis[id[G[u][k]]]){
                    vis[id[G[u][k]]] = 1;
                    rG[i].push_back(id[G[u][k]]);
                }
            }
        }
    }
    int t = 0, flag = 0;
    for (int i = 0; i < counts; i++){
        if (rG[i].size() == 0){
            flag++;
            t = i;
        }
    }
    if (flag == 1)cout << tG[t].size() << endl;
    else cout << 0 << endl;
}