articulation point

void dfs(ll u, ll p)
{
    vis[u] = 1;
    lvl[u] = low[u] = 1 + lvl[p];
    ll child = 0;
    for(ll v : g[u])
    {
        if(v == p) continue;
        if(!vis[v])
        {
            child++;
            dfs(v, u);
            if(u == 1) { // root case
                ap[u] = (ap[u] | child > 1);
            }
            else if(low[v] >= lvl[u]) {
                ap[u] = 1;
            }
            low[u] = min(low[u], low[v]);
        }
        else {
            // back edge (u, v)
            // update with lvl[v], not low[v] because we can use only 1 back edge
            low[u] = min(low[u], lvl[v]);
        }
    }
}