Linked List Cycle

 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)
13             return false;
14         ListNode* fast = head->next;
15         ListNode* slow = head;
16         while(fast) {
17             if(fast == slow)
18                 return true;
19             if(fast->next&&fast->next->next){//短路的技巧
20                 fast = fast->next->next;
21                 slow = slow->next;
22             }
23             else
24                 return false;
25         }
26         return false;
27     }
28 };