109. Convert Sorted List to Binary Search Tree

 1     public TreeNode toBST(ListNode head, ListNode tail){
 2         if (head == tail) return null;
 3         ListNode slow = head,fast=head;
 4         while (fast!=tail && fast.next!=tail)
 5         {
 6             slow = slow.next;
 7             fast = fast.next.next;
 8         }
 9         TreeNode tree = new TreeNode(slow.val);
10         tree.left = toBST(head,slow);
11         tree.right = toBST(slow.next,tail);
12         return tree;
13     }
14     
15     public TreeNode sortedListToBST(ListNode head) {
16         if(head==null) return null;
17         return toBST(head,null);        
18     }