题解 | 判断链表中是否有环

import java.util.*;
/**
 * 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) {
        HashMap<ListNode,Integer> map = new HashMap();
        if(head == null || head.next == null){
            return false;
        }
        ListNode now = head;
        while(now != null){
            if(map.get(now) != null){
                return true;
            }else{
                map.put(now,now.val);
                now = now.next;
            }
        }
        return false;
    }
}