LeetCode:141 环形链表（哈希表）

/**
 * 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> s = new HashSet<>();
    
        while(head!=null)
        {
            if(s.contains(head)){
                return true;
            }
            else{
                s.add(head);
                head = head.next;
            }
        }

        return false;
    }
}