Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def sortedListToBST(self, head):
        """
        :type head: ListNode
        :rtype: TreeNode
        """
        if head:
            pre = None
            slow = fast = head
            while fast and fast.next:
                pre = slow
                slow = slow.next
                fast = fast.next.next
            root = TreeNode(slow.val)
            if pre:
                pre.next = None
                root.left = self.sortedListToBST(head)
            root.right = self.sortedListToBST(slow.next)
            return root