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: [] Example 3: Input: head = [0] Output: [0] Example 4: Input: head = [1,3] Output: [3,1] Constraints: The number of nodes in head is in the range [0, 2 * 104]. -10^5 <= Node.val <= 10^5