[leetcode]Convert Sorted List to Binary Search Tree

 1 public class Solution {
 2     public TreeNode sortedListToBST(ListNode head) {
 3         if(head == null) return null;
 4         if(head.next == null) return new TreeNode(head.val);
 5         ListNode hhead = new ListNode(0);
 6         hhead.next = head;
 7         ListNode fast = hhead, slow= hhead;
 8         while(fast != null && fast.next != null){
 9             fast = fast.next.next;
10             slow = slow.next;
11         }
12         TreeNode root = new TreeNode(slow.val);
13         ListNode right = slow.next;
14         ListNode pre = hhead;
15         while(pre != null && pre.next != slow){
16             pre = pre.next;
17         }
18         pre.next = null;
19         ListNode left = hhead.next;
20         root.left = sortedListToBST(left);
21         root.right = sortedListToBST(right);
22         return root;
23     }
24 }