快慢指针。
快慢指针相等while结束。快指针出现null时返回false
public boolean hasCycle(ListNode head) {
if(head==null||head.next==null)
return false;
ListNode fast = head.next;
ListNode slow=head;
while(fast!=slow)
{
if(fast.next==null||fast.next.next==null)
return false;
fast=fast.next.next;
slow=slow.next;
}
return true;
}```