109. Convert Sorted List to Binary Search Tree

public TreeNode SortedListToBST(ListNode head) {
        TreeNode tree = null;
        if(head == null) return tree;
        if(head.next == null) return new TreeNode(head.val);
        //find the middle pionter;
        var walker = head;
        var runner = head;
        var prev = new ListNode(-1);
        while(runner != null && runner.next != null)
        {
            prev = walker;
            walker = walker.next;
            runner = runner.next.next;
        }
        prev.next =null;
        //root
        tree = new TreeNode(walker.val);
        //subtree
        tree.left = SortedListToBST(head);
        tree.right = SortedListToBST(walker.next);
        return tree;
    }