判断仅有指针域的单链表是否有环且怎么找到环

链表结构如下：

slist* FindLoopPort(slist *head){    slist *slow = head, *fast = head;    while ( fast && fast->next )     {        slow = slow->next;        fast = fast->next->next;        if ( slow == fast ) break;    }    if (fast == NULL || fast->next == NULL)        return NULL;    slow = head;    while (slow != fast)    {         slow = slow->next;         fast = fast->next;    }    return slow;}