https://leetcode.cn/problems/c32eOV/

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我们使用两个指针，fast 与 slow。它们起始都位于链表的头部。随后slow 指针每次向后移动一个位置，而fast 指针向后移动两个位置。如果链表中存在环，则fast 指针最终将再次与slow 指针在环中相遇。

struct ListNode *detectCycle(struct ListNode *head) {
    struct ListNode *fast = head, *slow = head;

    while(fast && fast->next)
    {
        slow = slow->next;
        fast = fast->next->next;

        if(fast == slow)
        {
            struct ListNode * meet = slow;
            while(head != meet)
            {
                head = head->next;
                meet = meet->next;
            }
            return meet;
        }
        
    }

    return NULL;
}