cut points

vector<vector<int>> g;
vector<int> used;
vector<int> up;
vector<int> tin;
vector<int> ans;
int timer = 0;

void dfs(int v, int parent) {
    tin[v] = up[v] = timer++;
    used[v] = 1;
    int cnt = 0;
    for (auto u : g[v]) {
        if (u == parent) {
            continue;
        }
        if (used[u]) {
            up[v] = min(up[v], tin[u]);
        } else {
            cnt += 1;
            dfs(u, v);
            up[v] = min(up[v], up[u]);
            if (up[u] >= tin[v] && parent != -1) {
                ans.push_back(v + 1);
            }
        }
    }
    if (parent == -1 && cnt > 1) {
        ans.push_back(v + 1);
    }
}