explore col 1
        1--place, explore col 2
                1
                2
                3--place, explore col 3
                        1
                        2
                        3
                        4
                4--place, explore col 3
                        1
                        2--place, explore col 4
                                1
                                2
                                3
                                4
                        3
                        4
        2--place, explore col 2
                1
                2
                3
                4--place, explore col 3
                        1--place, explore col 4
                                1
                                2
                                3--place, explore col 5
                                        done!