# Linked List Cycle

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

> Given a linked list, determine if it has a cycle in it.
>
> Follow up:
>
> Can you solve it without using extra space?

## Thoughts

有环意味着什么？想想我们跑圈时，跑得慢的能被跑得快的追上。利用这个思想。

## Code

```cpp
/**
 * 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) {
        ListNode slow = head, fast = head;

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

## Analysis

Errors:

1. 让跑得慢起点在前。导致即使无环也追上了
2. 忘了写node != null作为循环条件。

时间复杂度O(n)
