Convert Sorted Array to Binary Search Tree LeetCode solution in Python

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class Solution:
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    def formNodes(self, nums, l, r):
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        if l > r:
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            return None
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        else:
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            mid = l + (r - l) // 2
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            node = TreeNode(nums[mid])
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            node.left = self.formNodes(nums, l, mid - 1)
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            node.right = self.formNodes(nums, mid + 1, r)
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            return node
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    def sortedArrayToBST(self, nums: List[int]) -> Optional[TreeNode]:
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        return self.formNodes(nums, 0, len(nums) - 1)