/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
bool hasCycle(struct ListNode *head) {
struct ListNode*slow=head;
struct ListNode*fast=head;
while(fast&&fast->next)//同中间节点一样
{
    fast=fast->next->next;
    slow=slow->next;
    if(fast==slow)
    return true;
}
return false;
}

 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode *detectCycle(struct ListNode *head) {
    struct ListNode*slow=head;
    struct ListNode*fast=head;
    struct ListNode*mid=NULL;
    struct ListNode*init=head;
    while(fast&&fast->next)
    {
        fast=fast->next->next;
        slow=slow->next;
        if(fast==slow)
        {
            mid=fast;//mid在环里面，而init在环外面，相遇的节点就是进入的节点
            while(mid!=init)
            {
                mid=mid->next;
                init=init->next;
            }
            return mid;
        }
    }
    return NULL;
}