力扣——环形链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode *detectCycle(struct ListNode *head) 
{
    struct ListNode* fast=head;
    struct ListNode* slow=head;
    while(fast&&fast->next)
    {
        slow=slow->next;
        fast=fast->next->next;
        if(fast==slow)//证明有环
        {
            struct ListNode* meetNode=slow;
            while(meetNode!=head)//找入环的第一个结点
            {
                meetNode=meetNode->next;
                head=head->next;
            }
            return head;
        }
    }//循环结束，证明无环
    return NULL;
}