reverse_groups.cpp

node *reverse(struct node *head, int k)
{ 
    node* curr=head, *prev=NULL, *last=NULL;
    while(curr){
        int count=0;
        node *h = curr;
        // h->next=NULL;
        while(curr && count<k){
            node* next = curr->next;
            curr->next = prev;
            prev=curr;
            curr=next;
            if(next)
            next = next->next;
            count++;
        }
        if(!last){
            head = prev;
        }
        else{
            last->next = prev;
        }
        last = h;
        prev = NULL;
    }
    return head;

}