Longest ZigZag Path in a Binary Tree LeetCode solution in Python

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# Runtime: 437 ms (Top 91.25%) | Memory: 59.8 MB (Top 86.06%)
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class Solution:
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    def longestZigZag(self, root: Optional[TreeNode]) -> int:
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        self.res = 0
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        def helper(root):
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            if root is None:
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                return -1, -1
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            leftRight = helper(root.left)[1] + 1
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            rightLeft = helper(root.right)[0] + 1
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            self.res = max(self.res, leftRight, rightLeft)
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            return leftRight, rightLeft
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        helper(root)
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        return self.res