# AVL tree balance property states for the two subtrees of a node, their height can differ at most one. Why can't it be zero?

I was thinking that if they were equal, say they are required to be zero this would be enforce the balance property more effectively. Can anyone explain why 1 is a satisfactory rather than just them being 0?

• If you enforce a balance difference of zero, your tree can't hold an arbitrary number of elements anymore (only $2^k-1$ nodes, I think). For example, how would you arrange your tree when it has only two nodes? – siracusa Aug 20 '19 at 1:37

What matters is that the height of the tree is limited by $$O(\log n)$$. As long as you maintain that property your searches will remain efficient.