Return ``True`` if all the leaves
        in the subtree rooted at this node
        are at the same level.

        :rtype: bool

def is_pleasant(self):
        """
        Return ``True`` if all the leaves
        in the subtree rooted at this node
        are at the same level.

        :rtype: bool
        """
        levels = sorted([n.level for n in self.leaves])
        return levels[0] == levels[-1]