Binary Tree Level Order Traversal LeetCode solution in Python

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# Definition for a binary tree node.
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# class TreeNode:
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#     def __init__(self, val=0, left=None, right=None):
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#         self.val = val
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#         self.left = left
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#         self.right = right
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class Solution:
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    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
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        ret = []
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        next_levels = [[root]]
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        for level in next_levels:
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            curr_lv = []
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            next_lv = []
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            for node in level:
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                if not node:
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                    continue
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                curr_lv.append(node.val)
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                next_lv.append(node.left)
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                next_lv.append(node.right)
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            if curr_lv:
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                ret.append(curr_lv)
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            if next_lv:
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                next_levels.append(next_lv)
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        return ret