A. head==NULL B. head->next==NULL C. head->next==head D. head!=NULL
A. q=H;while(q!=p)q=q->next; B. q=H;while(q->next!=p)q=q->next; C. q=H->next;while(q!=p)q=q->next; D. q=H->next;while(q->next!=p)q=q->next;
A. p->next=s; s=p->next; B. p->next=s; s->next=p->next; C. s->next=p->next; p->next=s; D. s->next=p; p->next=s;
A. len=0,p=H;while(p->next!=NULL){len++;p=p->next;} B. len=0,p=H->next;while(p->next!=NULL){len++;p=p->next;} C. len=1,p=H;while(p!=NULL){p=p->next;len++;} D. len=1,p=H->next;while(p->next!=NULL){p=p->next;len++;}