(1). 假如队列未满，现有变量data需要入队,请写出表达式;

if( (tail+1)%SEQLEN != head )
{
    seqn[tail] = data;
    tail = (tail+1)%SEQLEN;
}

(2). 假如队列未空，现在需要从队列取一个元素并赋值给变量data，请写出表达式;

if( head != tail )
{
    data = seqn[tail];
    head = (head+1)%SEQLEN;
}

(3) 请写出队列为空的判断条件:

if(head == tail)
{
}

(3) 请写出队列满的判断条件:

if( (tail+1) % SEQLEN == head )
{
}

(4) 请写出清空队列的表达式

while(head != tail)
{
    (tail+1) % SEQLEN == head
}

(6)请写出计算队列中元素个数的表达式:

(tail - head + SEQLEN) % SEQLEN

(7)队列最多可以存放几个元素:

SEQLEN - 1

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插入到 prev 和 next 中间

new->next = next;
prev->next =new;

删除 prev 和 next中间那个

new = prev->next;
prev->next = new->next;
free(new);
new = NULL;

插入到队尾

new->next = NULL;
head->next = new;
head = head->next;

删除

prev = entry->next;
entry->next = NULL;
free(entry);
entry = NULL;

判断是否为空

head->next = head->prev;