I had to use recursion to build a tree in AoC '22 day 7. I needed to have both a mutable stack of current tree nodes (`cwd` if you like) and the tree structure.

I had to use recursion to build a tree in AoC '22 day 7. I needed to have both a mutable stack of current tree nodes (`cwd` if you like) and the tree structure. Or equivalently a doubly-linked tree, where nodes point at their parent. It was a bit ugly. In C++ you just write it. I get why it's hard in Rust, just saying, it really does crop up.