判断链表中是否有环

bool hasCycle(struct ListNode *head) {
    if(head==NULL)
    return false;
    else{
    struct ListNode *fast,*slow;
    fast=head->next;
    slow=head;
    while(fast!=NULL){
        if(slow==fast)
        return true;
        slow=slow->next;
        fast=fast->next;
        if(fast!=NULL)
        fast=fast->next;
    }
    if(fast==NULL)
    return false;
    else
    return true;
    }
}