Create a method size that accepts a cs125.trees.BinaryTree and returns the number of nodes it contains. size accepts a BinaryTree<*>?, that is a nullable BinaryTree that can contain any kind of values. You'll want to count recursively, identifying both a base case and a recursive step.

For reference, cs125.trees.BinaryTree is defined like this:

Don't overthink this! Like many recursive algorithms, the solution is elegant and simple: 4 lines total if you do it right. You'll also need to import cs125.trees.BinaryTree for this and similar problems.