Advent of Code - adventofcode.com

That's true, but think about when you would naturally process an entry in the stack:

lifo = [n]
seen = {n}
while lifo:
    node = lifo.pop()
    yield node # <- parent appears before child in iteration
    for c in node.children:
       if c not in seen:
           seen.add(c)
           lifo.append(c)

If you care about a strict left-to-right order of children in a tree-like structure you would walk over reversed(node.children).