判断带头结点的双循环链表是否对称

头结点：headbool BL(){    bool isOK = false;    struct DulNode * start, * p;    int len = 1;    start = head->next;    p = start->next;    while(p != start)    {        len++;        p = p->next;    }    int l = len/2;    while(l--)    {        l1 = len/2;        p1 = p;        q = p1->next;        while(l1--)        {            if(p1.data != q.data)                break;            p1 = p1->next;            q = q->next;        }        if(l1 == 0)        {            isOK = true;            break;        }        p = p->next;    }    return isOK;}