revrese node in k group
ListNode* reverseKGroup(ListNode* head, int k) {
int count = 0;
ListNode* prevptr= NULL;
ListNode* currptr= head;
ListNode* nextptr;
ListNode *check = head;
while(check) {
count++;
check = check->next;
}
if(count < k) return currptr;
else count = 0;
while(currptr!=NULL && count<k){
nextptr = currptr->next;
currptr->next=prevptr;
prevptr=currptr;
currptr=nextptr;
count ++;
}
if(nextptr!=NULL)
head ->next = reverseKGroup(nextptr, k);
return prevptr;
}