# Problem 142: Linked List Cycle II

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

## 思路

![](/files/-Lpv9w2vn71ujLiAv0VZ)

* 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;
   }
   ```


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