Nested Filesystem Size (Boxes Within Boxes)

Question

## Problem

Model a filesystem of folders containing files and sub-folders; compute total size of any node. Solve iteratively, then improve recursively.

## Example
```
folder{ a.txt(10), sub{ b.txt(5) } } -> 15
```

## Constraints
- Arbitrary nesting depth

## What Autodesk looks for
Clean recursive traversal and handling of deep nesting; discuss stack depth limits.