模板，无向图强连通缩点，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 ++;
}
}
//重新建图啦
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]);
}