Reverse a Linked List in groups of given size (GFG)

struct node *reverse (struct node *head, int k)
    { 
        // Complete this method
        // Complete this method
        if(head == nullptr)
        return head;
        struct node *p = head, *q = nullptr, *r;
        int c = 1;
        while(p && c <= k) {
            r = q;
            q = p; 
            p = p -> next;
            q -> next = r;
            c++;
        }
        head -> next = reverse(p, k);
        return q;
    }