点双连通分量

#include "../header.cpp"
vector <vector<int>> A;
vector <int> V[MAXN];
stack  <int> S;
int D[MAXN], L[MAXN], o; bool I[MAXN];
void dfs(int u, int f){
  D[u] = L[u] = ++ o; I[u] = true, S.push(u);
  int s = 0;
  for(auto &v : V[u]) if(v != f){
    if(D[v]){
      if(I[v])   L[u] = min(L[u], D[v]);
    } else {
      dfs(v, u), L[u] = min(L[u], L[v]), ++ s;
      if(L[v] >= D[u]){
        vector <int> T;
        while(S.top() != v){
          int t = S.top(); S.pop();
          T.push_back(t), I[t] = false;
        }
        T.push_back(v), S.pop(), I[v] = false;
        T.push_back(u);
        A.push_back(T);
      }
    }
  }
  if(f == 0 && s == 0)
    A.push_back({u}); // 孤立点特判
}