nulptr

    fn get_depth(root_opt: &Option<Box<TreeNode>>) -> usize {
        let root = match root_opt {
            Some(root) => root,
            None => return 0;
        };

        1 + std::cmp::max(get_depth(root.borrow().left), get_depth(root.borrow().right))
    }

  - how you test your code after you finish coding

  - what clarifying questions you asked to tease out a concrete question

  - what edge cases you thought of and how you handled them

  - how you handle bugs if they appear in the code

  - what solutions you presented and what their tradeoffs were, and why you decided to use a particular solution

  - whether your code was idiomatic

  - whether you used clean abstractions

  - maybe also comments and variable naming