# 141.Linked-List-Cycle

## 141. Linked List Cycle

## 题目地址

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

## 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 = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Follow up:
Can you solve it using O(1) (i.e. constant) memory?
```

## 代码

### Approach #1 HashSet

```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) {
      Set<ListNode> nodesSeen = new HashSet<>();
      while (head != null) {
          if (nodesSeen.contains(head)) {
              return true;
          } else {
              nodesSeen.add(head);
          }
          head = head.next;
      }
      return false;
  }
}
```

### Approach #2 Two Pointers

```java
class Solution {
  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;
}
```


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