class Solution {
    public List<List<Integer>> criticalConnections(int n, List<List<Integer>> connections) {
        Set<List<Integer>>[] adj = new HashSet[n];
        for (int i = 0; i < n; i++) adj[i] = new HashSet<List<Integer>>();
        for (List<Integer> conn: connections) adj[conn.get(0)].add(conn);

        Set<List<Integer>> critical = new HashSet<>();
        for (List<Integer> conn: connections) q.add(conn);

        Set<List<Integer>> q = new HashSet<>();
        for (List<Integer> conn: connections) q.add(conn);

        while (!q.isEmpty()) {
            List<Integer> edge = q.iterator().next();
            q.remove(edge);

            System.out.println("Picked up edge " + edge.get(0) + "-" + edge.get(1));

            Map<Integer, List<Integer>> visited = new HashMap<>();
            visited.put(edge.get(0), edge);
            int node = edge.get(1);
            for (;;) {
                if (adj[node].isEmpty()) break;

                List<Integer> nextEdge = adj[node].iterator().next();
                int nextNode = nextEdge.get(1);

                if (visited.containsKey(nextNode)) {
                    System.out.println("Cycle at " + nextNode);

                    node = nextNode;
                    do {
                        List<Integer> edge = visited.get(node);
                        q.remove(edge);
                        adj[node].remove(edge);
                        critical.remove(edge)
                        node = edge.get(1);
                    } while (node != nextNode);

                    break;
                }
                visited.put(node, nextEdge);
                node = nextNode;
            }
        }

        return critical;
    }
}