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 };