0515：在每个树行中找最大值（★）

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def largestValues(self, root: TreeNode) -> List[int]:
    res, level = [], [root] if root else []
    while level:
        res.append(max(node.val for node in level))
        level = [child for node in level for child in [node.left, node.right] if child]
    return res