
Create a method countGreaterThan that accepts a BinaryTree<Int>? and an Int and returns a count of the
number of nodes in the tree that contain a value greater than the passed value.
If the passed tree is null you should return 0.
For reference, cs125.trees.BinaryTree<T> is defined like this:
As a result, tree.value is an Int which you can compare to the threshold value in the usual
way.
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