141. Linked List Cycle

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

/**
 * 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>  set =new HashSet<>();
         while(head != null) {
             if(set.contains(head))
                 return true;
             else 
                 set.add(head);
             head = head.next; //需要注意是，此操作会改变head里的元素?
         }
         return false;
    }
}