<span>使用Tarjan进行缩点无向图</span>

int From[maxn],Laxt[maxn],To[maxn<<2],Next[maxn<<2],cnt;
int low[maxn],dfn[maxn],times,q[maxn],head,scc_cnt,scc[maxn];
vector <int> G[maxn];
int dis[maxn],S,T,ans;
void add(int u,int v)
{
Next[++cnt]=Laxt[u]; From[cnt]=u;
Laxt[u]=cnt; To[cnt]=v;
}
void tarjan(int u,int fa)
{
dfn[u]=low[u]=++times;
q[++head]=u;
for(int i=Laxt[u];i;i=Next[i]){
if(To[i] <mark>fa) continue;
if(!dfn[To[i]]) {
tarjan(To[i],u);
low[u]=min(low[u],low[To[i]]);
}
else low[u]=min(low[u],dfn[To[i]]);
}
if(low[u]</mark>dfn[u]){
scc_cnt++;
while(true){
int x=q[head--];
scc[x]=scc_cnt;
if(x==u) break;
}
}
}
void init()
{
memset(Laxt, 0, sizeof(Laxt));
cnt = 0;
}
int main()
{
init();
int N,M,u,v,i,j;
scanf("%d%d",&N,&M);
for(i=1;i<=M;i++){
scanf("%d%d",&u,&v);
add(u,v); add(v,u);
}
tarjan(1,0);
for(i=1;i<=N;i++){
for(j=Laxt[i];j;j=Next[j]){
if(scc[i]!=scc[To[j]])
G[scc[i]].push_back(scc[To[j]]);
}
} </int>

return 0;

}