Reverse a Linked List in groups of given size. (Function Problem)

problem id:-http://practice.geeksforgeeks.org/problems/reverse-a-linked-list-in-groups-of-given-size/1

code:--
/*
Please note that it's Function problem i.e.
you need to write your solution in the form Function(s) only.
Driver Code to call/invoke your function would be added by GfG's Online Judge.*/


/*
  Reverse a linked list
  The input list will have at least one element
  Return the node which points to the head of the new LinkedList
  Node is defined as
  struct node
  {
     int data;
     struct Node *next;
  }
*/
node *rev_k(node* head,int k)
    {
    if(head==nullptr)   return head;
    node *prev,*curr,*temp;
    prev=head;
    curr=prev->next;
    while(--k >0 && curr)
        {
        if(curr->next)  temp=curr->next;
        else    temp=nullptr;
        curr->next=prev;
        prev=curr;
        curr=temp;
        }
    return prev;
    }
struct node *reverse (struct node *head, int k)
{
if(head==nullptr)   return head;
node *f_node,*last_node;
f_node=head;
last_node=head;
int j=k;
while(j-- >0 && last_node)    last_node=last_node->next;
node *last=rev_k(head,k);
f_node->next=reverse(last_node,k);
return last;

}