Exercise 1: Pre-Order and Post-Order Traversal of a File System — Possible Solution
====================================================================

GIVEN
------------------------------
root -> {docs, src}
docs -> {readme.txt}
src -> {main.py, utils.py}

PRE-ORDER TRAVERSAL (visit BEFORE children)
------------------------------
Visit root first, then fully traverse its first child (docs)
before moving to its second child (src).

root, docs, readme.txt, src, main.py, utils.py

POST-ORDER TRAVERSAL (visit AFTER children)
------------------------------
Fully traverse docs's own children before docs itself; fully
traverse src's own children before src itself; visit root only
once everything beneath it is done.

readme.txt, docs, main.py, utils.py, src, root

WHICH ONE MATCHES A REAL DIRECTORY LISTING
------------------------------
Pre-order matches how a typical directory-listing tool prints a
folder before descending into it - "root" is printed first, then
"docs" is printed and its contents shown, then "src" is printed
and its contents shown. This is exactly this chapter's own
definition of pre-order: visiting a node before any of its
children, which is precisely "print the folder name, then show
what's inside it" rather than the reverse.

WHY THIS WORKS AS AN ANSWER
------------------------------
Both traversals are produced by consistently applying this
chapter's own visit-timing rule for every node, not just the root
- docs and src are each expanded fully (all their own children
handled) using the same before/after rule recursively, and the
real-world match is justified by pointing at the specific
definition (visit-before-children) rather than just asserting
pre-order "looks like" a directory listing.