141. 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 == NULL || head->next == NULL){
13             return false;
14         }
15         
16         ListNode* fast = head;
17         ListNode* slow = head;
18         
19         while(fast->next != NULL && fast->next->next != NULL)
20         {
21             fast = fast->next->next;
22             slow = slow->next;
23             if(fast  == slow){
24                 return true;
25             }
26         }
27         return false;
28     }
29 };