node* reverseLinkedList (node* & head) {
  //empty list or single node
  if (head == NULL || head -> next == NULL) {
    return head;
  }
  node* prev = NULL;
  node* curr = head;
  node* forword = NULL;
  while (curr != NULL) {
    forword = curr -> next;
    curr -> next = prev;
    prev = curr;
    curr = forword;
  }
  return prev;
}