142. 环形链表 II

public class Solution {
    public ListNode detectCycle(ListNode head) {
        //如果没有长度或者长度是1自然不是环
        if(head == null || head.next == null){
             return null;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while(slow != fast){
            if(fast == null || fast.next == null){
                return null;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        //slow 继续走，fast从head开始重新走，每次走一步直到两个相遇
        fast = head;
        while (slow != fast){
            slow = slow.next;
            fast = fast.next;
        }
        fast = head;
        while (slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }
        return fast;
    }
}