Convert Sorted List to Binary Search Tree

recursion programming
 1 public class Solution {
 2     public TreeNode sortedListToBST(ListNode head) {
 3         // IMPORTANT: Please reset any member data you declared, as
 4         // the same Solution instance will be reused for each test case.
 5         if(head == null)
 6             return null;
 7         ListNode tmp = head;
 8         int count = 0;
 9         while(tmp !=null){
10             tmp = tmp.next;
11             count++;
12         }
13         return buildBST(head, count);
14     }
15     
16     private TreeNode buildBST(ListNode head, int n)
17     {
18         if(head == null || n==0)
19             return null;
20         if(n == 1)
21             return new TreeNode(head.val);
22         int boundary = n/2;
23         ListNode tmp = head;
24         for(int i = 0; i < boundary; i++)
25             tmp = tmp.next;
26         TreeNode mynode = new TreeNode(tmp.val);
27         mynode.left = buildBST(head, boundary);
28         mynode.right = buildBST(tmp.next, n-boundary-1);
29         return mynode;
30     }
31 }