判断链表中是否有环

public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode head1=head;
        ListNode head2=head;
        if(head==null){
            return false;
        }
        while(head1!=null&&head1.next!=null){
            head2=head2.next;
            head1=head1.next.next;
            if(head1==head2)
            return true;
        }
        
        return false;

    }
}