class Solution { public: int minReorder(int n, vector<vector<int>>& connections) { vector<vector<int>> graph(n); for (const auto& conn : connections) { graph[conn[0]].push_back(conn[1]); graph[conn[1]].push_back(-conn[0]); // - := conn[0] -> conn[1] } return dfs(graph, 0, -1); } private: int dfs(const vector<vector<int>>& graph, int u, int parent) { int change = 0; for (const int v : graph[u]) { if (abs(v) == parent) continue; if (v > 0) ++change; change += dfs(graph, abs(v), u); } return change; } };