> For the complete documentation index, see [llms.txt](https://liuyang89116.gitbook.io/my-leetcode-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://liuyang89116.gitbook.io/my-leetcode-book/chapter_4_linked_list/find-cycles/problem_142_linked_list_cycle_ii.md).

# Problem 142: Linked List Cycle II

> <https://leetcode.com/problems/linked-list-cycle-ii/>

## 思路

![](https://1241747088-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Lpv9LvBSlFaukf_ALqh%2F-Lpv9NPw3Ji1X5Vk8CES%2F-Lpv9w2vn71ujLiAv0VZ%2FLinkedListCycle.jpg?generation=1569729533433178\&alt=media)

* 开始还是快慢指针的东西
* 发现有cycle之后（也就是说跳出了第一个while循环），head和slow同时出发，下次，head和slow\.next相遇的时候，就是cycle的位置
* **无他，记住就好了**

```java
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        if (head == null || head.next == null) {
            return null;
        }

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

        while (head != slow.next) {
            head = head.next;
            slow = slow.next;
        }

        return head;
    }
}
```

## 易错点

1. 判断位置是看 head 和 slow\.next 的相遇点

   ```java
   while (head != slow.next) {
          head = head.next;
          slow = slow.next;
   }
   ```