【leetcode】Convert Sorted List to Binary Search Tree

TreeNode *buildTree(ListNode* head, ListNode *end){
        if(head == NULL || head == end)
        return NULL;
        ListNode* fast = head;
        ListNode* slow = head;
        //get the middle 
        //notice: != end, not != NULL
        while(fast != end && fast->next != end){
            fast = fast->next->next;
            slow = slow->next;
        }
        TreeNode *root = new TreeNode(slow->val);
        root->left = buildTree(head, slow);
        root->right = buildTree(slow->next, end);
        return root;
    }
    TreeNode *sortedListToBST(ListNode *head) {
        if(head == NULL) 
        return NULL;
        return buildTree(head, NULL);
    }