class Solution { public: long long countPairs(int n, vector<vector<int>>& edges) { vector<vector<int>> g(n); for (const auto& e : edges) { g[e[0]].push_back(e[1]); g[e[1]].push_back(e[0]); } vector<int> seen(n); long long cur = 0; function<void(int)> dfs = [&](int u) { ++cur; for (int v : g[u]) if (seen[v]++ == 0) dfs(v); }; long long ans = 0; for (int i = 0; i < n; ++i) { if (seen[i]++) continue; cur = 0; dfs(i); ans += (n - cur) * cur; } return ans / 2; } };