题解 | #判断链表中是否有环#

/**
 * 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) {
    	//方法一：集合判等
        if(head == null){
            return false;
        }
        
        ListNode head0 = head;
        HashSet<ListNode> hashSet = new HashSet<>();
        while(head0 != null){
            if(hashSet.contains(head0)){
                return true;
            }else{
                hashSet.add(head0);
            }
            head0 = head0.next;
        }
        
        return false;
    }
}