}

    private static List<String> visitCycle(
            Collection<Vertex> children, Map<Vertex, DfsState> stateMap, LinkedList<String> cycle) {
        for (Vertex v : children) {
            DfsState state = stateMap.putIfAbsent(v, DfsState.VISITING);
            if (state == null) {
                cycle.addLast(v.label);
                List<String> ret = visitCycle(v.children, stateMap, cycle);