Shouldn't it return a tree so that you can keep track of where each product belongs?
You can return whatever you like, but this way you are shifting the burden of tree iteration onto the consumer of your output. It also seems curious to state how you "avoid any sort of reverse or append of the lists" when you produce a tree.
That call pattern just confuses me even more
It shows that you are making 27 leaf calls to produce a collection of 27 items, so there's nothing to memoize since the lambdas contain information essential to the return value.