1192_c

class Solution {
public:
    vector<int> ids;
    vector<int> low;
    vector<bool> visited;
    int id;
    unordered_map<int,vector<int>> g;
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        
        for(int i = 0; i < connections.size(); i++){
            int a = connections[i][0];
            int b = connections[i][1];
            g[a].push_back(b);
            g[b].push_back(a);
            
        }
        id = 1;
        ids = vector<int>(n+1,-1);
        low = ids;
        visited = vector<bool>(n+1,0);
        vector<vector<int>> output;
        for(int i = 0; i < n; i ++){
            if(!visited[i])
                dfs(i,-1,output);
        }
        return output;
    }
    
    void dfs(int at, int parent, vector<vector<int>> &output){
        visited[at] = true;
        low[at] = ids[at] = id;
        id++;
        for(auto to:g[at]){
            if(to == parent) continue;
            if(!visited[to]){
                dfs(to,at,output);
                low[at] = min(low[at], low[to]);
                if(ids[at] < low[to])
                    output.push_back(vector<int>{at,to});
            }
            else
                low[at] = min(low[at], ids[to]);
        }
    }
};