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：

    TreeNode *sortedListToBST(ListNode *head, int len){
        if(len == 0){
            return NULL;
        }
        TreeNode *result = new TreeNode(0);
        if(len == 1){
            result->val = head->val;
            return result;
        }
        ListNode *listNode = head;
        for(int i = 0; i < len/2; i++)
            listNode = listNode->next;
        result->val = listNode->val;
        result->left = sortedListToBST(head, len/2);
        result->right = sortedListToBST(listNode->next, (len-1)/2);
        return result;
    }
    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 *listNode = head;
        int len = 0;
        while(listNode){
            len++;
            listNode = listNode->next;
        }
        return sortedListToBST(head, len);
    }