struct ListNode* reverseList(struct ListNode* head) {
    if(head==NULL)
    return head;
    struct ListNode*n1=NULL,*n2=head,*n3=head->next;
    while(n2)
    {
       n2->next=n1;
       n1=n2;
       n2=n3;
       if(n3) 
       n3=n3->next;
    }
    return n1;
}