Get descendants from a tree using a queue.

queue.py

def get_descendants(node, A):
    unexplored = deque([node])
    descendants = set()
    while unexplored:
        node = unexplored.popleft()
        descendants.add(node)
        children = A.getrow(node).indices
        for child in children:
            unexplored.append(child)

    return descendants