Given 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:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

  0
 / \

-3 9
/ /
-10 5

class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if(head == null) return null;
        return toBST(head,null);
    }
    private TreeNode toBST(ListNode head, ListNode tail){
        ListNode slow = head;
        ListNode fast = head;
        if(head == tail) return null;
        while(fast != tail && fast.next != tail){
            fast = fast.next.next;
            slow = slow.next;
        }
        TreeNode root = new TreeNode(slow.val);
        root.left = toBST(head, slow);
        root.right = toBST(slow.next, tail);
        return root;
    }
}