Return the height of the tree
        rooted at this node,
        that is, the difference between the level
        of a deepest leaf and the level of this node.
        Return ``1`` for a single-node tree,
        ``2`` for a two-levels tree, etc.

        :rtype: int

def height(self):
        """
        Return the height of the tree
        rooted at this node,
        that is, the difference between the level
        of a deepest leaf and the level of this node.
        Return ``1`` for a single-node tree,
        ``2`` for a two-levels tree, etc.

        :rtype: int
        """
        return max([n.level for n in self.subtree]) - self.level + 1