set<int>&. I don't see you modifying the set in dfs2, so it is safe to pass it as a reference. If not, the complexity is raised to $$$N^2$$$ due to there are $$$N$$$ dfs calls and the set is copied each time. (The logn part grows slower than the set copy part.)

you're copying the set object -> O(n) -> O(n2 * log(n))