def stats(self, result=None, counter=0):
        """
        Display the node stats info on specific depth in this dict.

        ::

            [
                {"depth": 0, "leaf": M0, "root": N0},
                {"depth": 1, "leaf": M1, "root": N1},
                ...
                {"depth": k, "leaf": Mk, "root": Nk},
            ]
        """
        if result is None:
            result = dict()

        if counter == 0:
            if len(self):
                result[0] = {"depth": 0, "leaf": 0, "root": 1}
            else:
                result[0] = {"depth": 0, "leaf": 1, "root": 0}

        counter += 1
        if len(self):
            result.setdefault(
                counter, {"depth": counter, "leaf": 0, "root": 0})
            for dict_tree in self.values():
                if len(dict_tree):  # root
                    result[counter]["root"] += 1
                else:  # leaf
                    result[counter]["leaf"] += 1
                dict_tree.stats(result, counter)

        return [
            collections.OrderedDict([
                ("depth", info["depth"]),
                ("leaf", info["leaf"]),
                ("root", info["root"]),
            ]) for info in
            sorted(result.values(), key=lambda x: x["depth"])
        ]
Display the node stats info on specific depth in this dict.

        ::

            [
                {"depth": 0, "leaf": M0, "root": N0},
                {"depth": 1, "leaf": M1, "root": N1},
                ...
                {"depth": k, "leaf": Mk, "root": Nk},
            ]