def preorder_children(children, root):
    result, pending = [], [root]
    while pending:
        node = pending.pop()
        result.append(node)
        pending.extend(reversed(children[node]))
    return result

children = {"orchard":["north", "south"], "north":["oak"], "south":[], "oak":[]}
print(preorder_children(children, "orchard"))