Linked List Cycle II

 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     ListNode *detectCycle(ListNode *head) {
12         
13         ListNode *fast = head;
14         ListNode *slow = head;
15         
16         if(fast == NULL || fast == NULL)
17             return NULL;
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             {
25                 slow = head;
26                 while(fast != slow)
27                 {
28                     fast = fast->next;
29                     slow = slow->next;
30                 }
31                 return fast;
32             }
33         }
34         return NULL;
35     }
36 };