数据结构：树🌲

时间复杂度：O(n)

空间复杂度：O(n)

代码实现：

class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        if not root: return []
        res = []
        q = [root]

        while q:
            curr = []
            for _ in range(len(q)):
                out = q.pop(0)
                if out.left:
                    q.append(out.left)
                if out.right:
                    q.append(out.right)
                curr.append(out.val)
            res.append(curr)

        return res