//快慢指针，若快指针走到头，则没有环；否则快慢指针相遇，有环；

public boolean hasCycle(ListNode head) {
        if(head==null ||head.next==null) return false;
        ListNode fast=head;
        ListNode slow=head;
        while(fast!=null && fast.next!=null){
            fast = fast.next.next;
            slow=slow.next;
            if(fast==slow) return true;
        }
        return false;
    }