快慢指针法判断链表中是否有环

    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;

        while (fast != null){
            if(fast.next != null){
                if(fast.next.next != null){
                    fast = fast.next.next;
                    slow = slow.next;

                    if(fast == slow){
                        return true;
                    }

                }else {
                    fast = null;
                }
            }else {
                fast = null;
            }
        }

        return false;
    }