Maybe you've never used a binary tree on the job to store information. If you've ever written a recursive function with two recursive calls, the call graph is

Maybe you've never used a binary tree on the job to store information. If you've ever written a recursive function with two recursive calls, the call graph is a binary tree. The interview problem "invert a binary tree", is more of a litmus test for understanding recursion, which is a fairly basic knowledge item for an SE.