int isExitsLoop(LinkList L) {
    LinkList fast, slow;
    fast = slow = L;
    while (fast && fast->next)
    {
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast)
        {
            break;
        }
    }
    return ((fast == NULL) || (fast->next == NULL));
}

while (fast&&slow)
{
　　if (fast->next==NULL)
　　    return slow ->data;
　　else if (fast->next!= NULL && fast->next->next== NULL)
　　    return (slow ->data + slow ->next->data)/2;
　　else
　　{
　　    fast= fast->next;
　    　fast= fast->next;
　　    slow = slow ->next;
　　}
　}

node* findLoopPort(node *head) {
    node *fast, *slow;
    fast = slow = 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;

// 查找单链表中倒数第K个结点  
ListNode * RGetKthNode(ListNode * pHead, unsigned int k) // 函数名前面的R代表反向  
{  
    if(k == 0 || pHead == NULL) // 这里k的计数是从1开始的，若k为0或链表为空返回NULL  
        return NULL;  
  
    ListNode * pAhead = pHead;  
    ListNode * pBehind = pHead;  
        for(int i=0;i<k-1;i++){  
           pAhead=pAhead->next;  
           if(pAhead==null)  return null;//当链表长度小于k时候，返回Null  
        }  
    while(pAhead->next != NULL)  // 前后两个指针一起向前走，直到前面的指针指向最后一个结点  
    {  
        pBehind = pBehind->next;  
        pAhead = pAhead->next;  
    }  
    return pBehind;  // 后面的指针所指结点就是倒数第k个结点  
}