141. 环形链表 LeetCode报错:runtime error: member access within null pointer of type 'struct ListNode'

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 class Solution {
 public:
     bool hasCycle(ListNode *head) {
         if(!head->next||!head)
             return false;
      ListNode *pre=NULL,* cur=head;

         while(cur)
         {
             ListNode* tmp=cur->next;
             cur->next=pre;
             pre=cur;
             cur=tmp;
         }
         return (pre==head);
     }
 };

这是出错代码，后来查找原因发现是第12行的空指针判断顺序有问题，应该为if(!head||!head->next)