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## 题目描述

Given a linked list, determine if it has a cycle in it.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = , pos = -1
Output: false
Explanation: There is no cycle in the linked list.
Can you solve it using O(1) (i.e. constant) memory?

## 代码

### Approach #1 HashSet

/**
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
Set<ListNode> nodesSeen = new HashSet<>();
return true;
} else {
}
}
return false;
}
}

class Solution {
return false;
}