I’m not going to apologize for being harsh. But I will link a resource to help you understand. http://cslibrary.stanford.edu/110/BinaryTrees.html
tmtvl's comment at https://news.ycombinator.com/item?id=41983916 shows an algorithm that requires unbounded space.
Traversing a mutable binary tree can be done in fixed space but is not easier to do with generalized forms of recursion than with tail recursion or imperative iteration.