2,p
        _.-'   `._                              3,r
       1          3,r                       _.-'   `--..___
      / \       _'   `--..___              1               5
     []  [2]   [2]           5        =>  / \          _.-' `._
                         _.-' `._        []  [3]      4        6
                        4        6                  _' \     _' \
                      _' \     _' \                [3]  [5] [5]  []
                     [3]  [5] [5]  []