Write a method is_balanced in LinkedBST which
determines if the tree is balanced or not. This method should
traverse the tree and check to see if any Node is the root of
an unbalanced subtree. If you find an unbalanced Node,
return False. If the whole tree is traversed and no
unbalanced Nodes are found, return True.
Hint: You can reuse some of your code from breadth in
this method to traverse the tree. Or if you prefer, you can
reuse some of the code from inorder.