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    def sortedListToBST(self, head: Optional[ListNode]):
        if not head:
            return None
        prev = node = mid = head
        while node and node.next:
            prev = mid
            mid = mid.next
            node = node.next.next
        if head == mid:
            return TreeNode(mid.val)
        prev.next = None # cul left part of List
        return TreeNode(mid.val, self.sortedListToBST(head), self.sortedListToBST(mid.next))