def sortedListToBST(self, head):
        """
        dfs
        No O(1) access.
        Bottom-up construction
        :param head: ListNode
        :return: TreeNode
        """
        if not head:
            return head

        self.current_node = head
        length = self.getLength(head)
        return self.sortedListToBST_dfs(0, length-1)
dataset/ETHPy150Open algorhythms/LeetCode/109 Convert Sorted List to Binary Search Tree.py/Solution.sortedListToBST/original