code: def height(self):
          
          return max([n.level for n in self.subtree]) - self.level + 1
docs: 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