> 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_141_linked_list_cycle.md).

# Problem 141: Linked List Cycle

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

## 思路

* 快慢指针问题：当快慢指针能相遇，就说明是有环的

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

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

## 易错点

1. 在while循环当中的退出条件

   ```java
   if (fast == null || fast.next == null) {
       return false;
   }
   ```

   只需要考虑快指针是否会越界就好了，因为如果快指针都没事儿的话，那么慢指针肯定没问题。 &#x20;
2. 有环的话是不会走到尽头的，会一直循环。