leetcode链表--5、linked-list-cycle(单链表是否有环)

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     bool hasCycle(ListNode *head) {
12        if(head == NULL)//记得空链表处理
13             return false;
14         ListNode *pSlow = head;
15         ListNode *pFast = head;
16         while(pFast->next != NULL && pFast->next->next != NULL)
17         {
18 
19             pSlow = pSlow->next;
20             pFast = pFast->next->next;
21             if(pSlow == pFast)//一定要移动完再判断
22                 return true;
23         }
24         return false;
25     
26     }
27 };