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

代码如下：

 1 /**
 2  * Definition for singly-linked list.
 3  * class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public boolean hasCycle(ListNode head) {
14         if(head==null)
15         return false;
16         
17         Map<ListNode,ListNode> map=new HashMap<>();
18         while(head!=null)
19         {
20             if(map.get(head)!=head)
21             map.put(head,head);
22             else return true;
23             head=head.next;
24         }
25         return false;
26     }
27 }