void swap_pair(node *head){
  if(head == NULL || head -> next == NULL) return ;
  node *cur,*prev,*next;
  cur = head -> next;
  prev = head;
  head = cur -> next;
  while(true) {
    next = cur -> next;
    if(next == NULL || next -> next == NULL){
      prev -> next = next;
      break;
    }
    cur -> next = prev;
    prev -> next = next -> next;
    prev = cur;
    cur = prev -> next;
  }
}