Linked List Cycle

 1 　　bool hasCycle(ListNode *head) {
 2         if(head==NULL)
 3             return false;
 4         ListNode *pBefore,*pLater;
 5         if(head->next)
 6             pBefore=head->next;
 7         else
 8             return false;
 9         pLater=head;
10         while(pBefore)
11         {
12             if(pBefore->next==pLater)
13                 return true;
14             pBefore=pBefore->next;
15             pLater=pLater->next;
16         }
17         return false;
18     }

Status: Time Limit Exceeded

超时，速度一样的话，是不对的，画个图就能很清晰的看出来。

 1     bool hasCycle(ListNode *head) {
 2         if(head==NULL)
 3             return false;
 4         if(head->next==NULL)
 5             return false;
 6         ListNode *pFast,*pSlow;
 7         pFast=head->next;
 8         pSlow=head;
 9         while(pFast&&pSlow){
10             if(pFast==pSlow)
11                 return true;
12             pFast=pFast->next->next;
13             pSlow=pSlow->next;
14         }
15         return false;
16     }

Status: Runtime Error

Last executed input:{1,2}, no cycle

这个又是怎么了？？？

分析,很不规范的分析，但大体意思是这么着，结合着错误提示看，

slow=1,fast=2

while(1&&2)

fast!=slow

fast=NULL->next;

错了吧，就在这里错的哇

 1     bool hasCycle(ListNode *head) {
 2         if(head==NULL||head->next==NULL)
 3             return false;
 4         ListNode *pFast,*pSlow;
 5         pFast=head;
 6         pSlow=head;
 7         while(pFast&&pFast->next){
 8             pSlow=pSlow->next;
 9             pFast=pFast->next->next;
10             if(pFast==pSlow)
11                 return true;
12         }
13         return false;
14     }

 1     bool hasCycle(ListNode *head) {
 2         if(head==NULL||head->next==NULL)
 3             return false;
 4         ListNode *pFast,*pSlow;
 5         pFast=head;
 6         pSlow=head;
 7         while(pFast->next){
 8             pSlow=pSlow->next;
 9             pFast=pFast->next->next;
10             if(pFast==pSlow)
11                 return true;
12         }
13         return false;
14     }

Status: Runtime Error

Last executed input:{1,2}, no cycle

我只是想省去一个判断信息：pFast&&pFast->next改成pFast->next

结合着错误提示看看

fast=1,slow=1

while(2)

slow=2;

fast=NULL;

while(NULL->next)

又出错了吧

 1 public:
 2     bool hasCycle(ListNode *head) {
 3         if(head->next==NULL)
 4             return false;
 5         ListNode *pFast,*pSlow;
 6         pFast=head;
 7         pSlow=head;
 8         while(pFast&&pFast->next){
 9             pSlow=pSlow->next;
10             pFast=pFast->next->next;
11             if(pFast==pSlow)
12                 return true;
13         }
14         return false;
15     }

第三行我把head==NULL给删了，呵呵，结果就

Submission Result: Runtime Error

Last executed input:

{}, no cycle