void add_in_back(Queue *q, int x){
    node *p = (node*) malloc(sizeof(node));
        p->data = x;
        p->next = NULL;
    if (!isempty(q)){
        p->pre = q->tail;
       (q->tail)->next=p;
    }else{
        p->pre = NULL;
        q->head=p;
    }
    q->tail = p;
}