if(head==NULL || k==1){
            return head;
        }
        ListNode* dummy=new ListNode(0);
         dummy->next=head;
        ListNode *cur=dummy,*pre=dummy, *nex=dummy;
       
        int cnt=0;
        while(cur->next!=NULL){
            cur=cur->next;
            cnt++;
        }
        while(cnt>=k){
            cur=pre->next;
            nex= cur->next;
            for(int i=1;i<k;i++){
                cout<<dummy->next->val;
                cur->next=nex->next;
                nex->next=pre->next;
                pre->next=nex;
                nex=cur->next;
                
            }
            pre=cur;
            cnt-=k;
        }
        return dummy->next;