# Linked List Cycle

## Question

[leetcode: Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/)\
[lintcode: Linked List Cycle](http://www.lintcode.com/en/problem/linked-list-cycle/)

```
Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?
```

## 题解思路

链表中的Two Pointers是一种很常用的思想，快慢两个指针，通过是否相遇，来判断链表中是否有环。相比于用HashMap的方法，优点是空间复杂度仅为O(1)，时间复杂度O(n)。

参考 LeetCode: <https://leetcode.com/articles/linked-list-cycle/>

## 源代码

```java
/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.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 (slow != fast) {
            if (fast == null || fast.next == null) {
                return false;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        return true;
    }
}
```