Given a linked list, reverse the nodes of a linked list k at a time and return its modified list. k is a positive integer and is less than or equal to the length of the linked list. If the number of

class Solution {
public:
    ListNode *reverseKGroup(ListNode *head, int k) {
        if (!head || !(head->next) || k < 2)
            return head;  

        // count k nodes
        ListNode *nextgp = head;
        for (int i = 0; i < k; i++)
            if (nextgp)
                nextgp = nextgp->next;
            else
                return head;  

        // reverse
        ListNode *prev = NULL, *cur = head, *next = NULL;
        while (cur != nextgp) {
            next = cur->next;
            if (prev)
                cur->next = prev;
            else
                cur->next = reverseKGroup(nextgp, k);
            prev = cur;
            cur = next;
        }
        return prev;
    }
};