分层遍历二叉树

def level_traverse_tree(node: TreeNode):
    ret = []
    if node is not None:
        cur = [node]
        while cur:
            tmp = []
            next_cur = []
            for item in cur:
                if item:
                    tmp.append(item.value)
                    next_cur.extend([item.l_node, item.r_node])
            if tmp:
                ret.append(tmp)
            cur = next_cur
    return ret