模板，無向圖強連通縮點，tarjan

int first[nMax],nxt[nMax],to[nMax],e;
void addadge(int u,int v) {
    to[e]=v;nxt[e]=first[u];first[u]=e;e++;
    to[e]=u;nxt[e]=first[v];first[v]=e;e++;
}
int low[nMax],dfn[nMax],dep,belong[nMax],vis[nMax],st[nMax],top,cnt;
void tarjan(int u,int E) {
    low[u] = dfn[u] = dep;dep++;
    vis[u] = 1;
    st[++top] = u;
    for(int i=first[u];i!=-1;i=nxt[i]) {
        if((E^1) == i) continue;
        if(!vis[to[i]]) {
            tarjan(to[i],i);
            low[u] = min(low[u],low[to[i]]);
        }else {
            low[u] = min(low[u],dfn[to[i]]);
        }
    }
    if(low[u] == dfn[u]){
        do{
            belong[st[top]] = cnt;
        }while(top && st[top--] != u);
        cnt ++;
    }
}
//重新建圖啦
for(int i=0;i<m;i++) {
    if(belong[edge[i].l] != belong[edge[i].r]) addadge(belong[adge[i].l],belong[edge[i].r]);
}