Convert Sorted List to Binary Search Tree

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

思路：
递归

代码：

 1     TreeNode *sortedListToBST(ListNode *head, int len){
 2         if(len == 0){
 3             return NULL;
 4         }
 5         TreeNode *result = new TreeNode(0);
 6         if(len == 1){
 7             result->val = head->val;
 8             return result;
 9         }
10         ListNode *listNode = head;
11         for(int i = 0; i < len/2; i++)
12             listNode = listNode->next;
13         result->val = listNode->val;
14         result->left = sortedListToBST(head, len/2);
15         result->right = sortedListToBST(listNode->next, (len-1)/2);
16         return result;
17     }
18     TreeNode *sortedListToBST(ListNode *head) {
19         // IMPORTANT: Please reset any member data you declared, as
20         // the same Solution instance will be reused for each test case.
21         if(head == NULL)
22             return NULL;
23         ListNode *listNode = head;
24         int len = 0;
25         while(listNode){
26             len++;
27             listNode = listNode->next;
28         }
29         return sortedListToBST(head, len);
30     }