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

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example 1:

    Input: head = [-10,-3,0,5,9]
    Output: [0,-3,9,-10,null,5]
    Explanation: One possible answer is [0,-3,9,-10,null,5], which represents the shown height balanced BST.

Example 2:

    Input: head = []
    Output: []

Constraints:

- The number of nodes in `head` is in the range `[0, 2 * 104]`.
- `-105 <= Node.val <= 105`