k个一组翻转链表

class  {
public:
    ListNode* reverse(ListNode* root) {
        ListNode* pre = NULL;
        ListNode* node = root;
        ListNode* next;
        while (node != NULL) {
            next = node->next;
            node->next = pre;
            pre = node;
            node = next;
        }
        return pre;
    }
    
    ListNode* reverseKGroup(ListNode* head, int k) {
        if (head == NULL || head->next == NULL || k == 1) return head;
        ListNode* result = new ListNode(-1);
        ListNode* r = result;
        
        while (head != NULL) {
            int i;
            ListNode* start = head;
            ListNode* next;
            for (i = 1; i < k && head != NULL; i++) head = head->next;
            if (i <= k && head == NULL) {
                r->next = start;
                break;
            } else {
                next = head->next;
                head->next = NULL;
                r->next = reverse(start);
                r = start;
                head = next;
            }
        }
        return result->next;
    }
};