0102：二叉树的层序遍历（★）

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class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        res,Q = [],[root] if root else []
        while Q:
            res.append([u.val for u in Q])
            Q = [c for u in Q for c in [u.left,u.right] if c]
        return res