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;
 *     }
 * }
 */
import java.util.*;
import java.util.Set;
public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head==null){
            return false;
        }
	  //用hashset或者hashset来判断是否有环
        HashSet<ListNode> set=new HashSet<>();
        while(head!=null){
		  //如果set包括该节点，说明有环
            if(set.contains(head)){
                return true;
            }
            set.add(head);
            head=head.next;
        }
        return false;
    }
}