binary tree balancing algorithm


1. start at top node


2. if left subtree is light (less items), push the top node down to the left child position,

	leaving a blank space at the top node position.

	otherwise, if right subtree is light, same action


3. bubble the space down the heavy subtree to the bottom, taking the heavy left/right path 

	at each point


4. repeat the process on the left subtree (treating left child as top node) and on the

	right subtree 



need to keep the number of items in the left subtree and the right subtree

