模板—v-dcc缩点

int dfn[MAXN],low[MAXN],num,root;
int stack[MAXN],top,cnt;
bool iscut[MAXN];
int new_id[MAXN],c[MAXN];
vector<int> dcc[MAXN];
void tarjan(int x)
{
    dfn[x]=low[x]=++num;
    stack[++top]=x;
    if(x==root&&!f(x)){dcc[++cnt].push_back(x);return;}
    int flag=0;
    for(int i=f(x);i;i=n(i))
    if(!dfn[v(i)])
    {
        tarjan(v(i));low[x]=min(low[x],low[v(i)]);
        if(low[v(i)]>=dfn[x])
        {
            flag++;
            if(x!=root||flag>1)iscut[x]=1;
            cnt++;int z;
            do{dcc[cnt].push_back(z=stack[top--]);}while(z!=v(i));
            dcc[cnt].push_back(x);
        }
    }
    else low[x]=min(low[x],dfn[v(i)]);
}
signed main()
{
    for(int i=1;i<=n;i++)
    if(!dfn[i]){root=i;tarjan(i);}
    num=cnt;
    for(int i=1;i<=n;i++)
    if(iscut[i])new_id[i]=++num;
    for(int i=1;i<=cnt;i++)
        for(int j=0;j<dcc[i].size();j++)
        {
            int x=dcc[i][j];
            if(iscut[x])
                 add2(i,new_id[x]),add2(new_id[x],i);
            else c[dcc[i][j]]=i;
        }
}