142. 环形链表 II

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/linked-list-cycle-ii
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

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逻辑不明白，只能是死记硬背，水平提高后，再来想想

    public ListNode detectCycle(ListNode head) {
        if(head == null) {
            return null;
        }

        ListNode slow = head;
        ListNode fast = head;

        while(fast != null && fast.next != null) {

            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast) {
                fast = head;
                while(slow!=fast) {
                    slow = slow.next;
                    fast = fast.next;
                }
                return fast;
            }
        }
        return null;
    }