141. Linked List Cycle

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

代码如下：

 /**
  * 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) {
         if(head==null)
         return false;

         Map<ListNode,ListNode> map=new HashMap<>();
         while(head!=null)
         {
             if(map.get(head)!=head)
             map.put(head,head);
             else return true;
             head=head.next;
         }
         return false;
     }
 }