Level Order of Binary Tree

7    while queue:
●        level_size = len(queue)
9        level = []
......
11            node, _ = queue.popleft()
12            level.append(node.val)
13            if node.left:
14                queue.append((node.left, 0))
15            if node.right:
16                queue.append((node.right, 0))
17        ans.append(level)
18    return ans