dependencies.py

def traverse(x: List[Tuple[int, int]]) -> Optional[List[int]]:
	if len(x) == 0:
		return []
	not_leaves = {l: True for (l, r) in x if r is not None}
	leaves = {i: True for j in x for i in j if i not in not_leaves and i is not None}
	new_x = [(l, None if r in leaves else r) for (l, r) in x if l in not_leaves]
	recursion = traverse(new_x)
	return [i for i in leaves] + recursion if len(new_x) < x and recursion is not None else None