Convert Sorted List to Binary Search Tree

recursion programming

public class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(head == null)
            return null;
        ListNode tmp = head;
        int count = 0;
        while(tmp !=null){
            tmp = tmp.next;
            count++;
        }
        return buildBST(head, count);
    }
    
    private TreeNode buildBST(ListNode head, int n)
    {
        if(head == null || n==0)
            return null;
        if(n == 1)
            return new TreeNode(head.val);
        int boundary = n/2;
        ListNode tmp = head;
        for(int i = 0; i < boundary; i++)
            tmp = tmp.next;
        TreeNode mynode = new TreeNode(tmp.val);
        mynode.left = buildBST(head, boundary);
        mynode.right = buildBST(tmp.next, n-boundary-1);
        return mynode;
    }
}