# Problem 142: Linked List Cycle II

> [https://leetcode.com/problems/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;
    }
    ```
