} else if (state == DfsState.VISITING) {
                // we are already visiting this vertex, this mean we have a cycle
                int pos = cycle.lastIndexOf(v.label);
                List<String> ret = cycle.subList(pos, cycle.size());
                ret.add(v.label);
                return ret;
            }
        }
        return null;
    }

    static class Vertex {

                } else if (state == DfsState.VISITING) {
                    // we are already visiting this vertex, this mean we have a cycle
                    int pos = cycle.lastIndexOf(v);
                    List<String> ret = cycle.subList(pos, cycle.size());
                    ret.add(v);
                    return ret;
                }
            }
        }
        return null;
    }

            } else if (state == DfsState.VISITING) {
                // we are already visiting this vertex, this mean we have a cycle
                int pos = cycle.lastIndexOf(v.label);
                List<String> ret = cycle.subList(pos, cycle.size());
                ret.add(v.label);
                return ret;
            }
        }
        return null;
    }

    static class Vertex {