判断链表中是否有环

快慢指针即可。

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



    }