题解 | #判断链表中是否有环#

import java.util.HashSet;
import java.util.Set;
/**
 * 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> hs = new HashSet<>();
        while(true)
        {
            if(!hs.add(head))
            {
                break;
            }
            if((head == null) || (head.next == null))
            {
                return false;
            }
            head = head.next;
        }
        return true;
    }
}