A binary tree is one way of preparing data, usually for sorting. Each node can have a left, right, or both, children.
A
/ \
B C
/ \
D E
“Inverting the tree” means swapping the children for each node, so that the order that the nodes are visited is reversed. Depending on whether you want to copy the tree or swap it in place then the algorithm is different. C++ provides iterators too, so providing a “order reversed” iterator can be done efficiently as well.
You’re going to have to visit every node and do at least one swap for every node, and an efficient algorithm won’t do much more than that. Bring unable to do it suggests that the student programmer doesn’t understand stacks or recursion yet, so they’ve more to learn.
If I believed that they were sincerely interested in trying to improve their product, then that would make sense. You can only improve yourself if you understand how your failings affect others.
I suspect however that Saltman will use it to come up with some superficial bullshit about how their new 6.x model now has a 90% reduction in addiction rates; you can’t measure anything, it’s more about the feel, and that’s why it costs twice as much as any other model.