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         if(head == NULL || head->next == NULL){13             return NULL;14         }15         16         ListNode* slow = head;17         ListNode* fast = head;18         19         while(fast->next != NULL && fast->next->next != NULL){20             fast = fast->next->next;21             slow = slow->next;22             23             if(fast == slow){24                 slow = head;25                 26                 while(slow != fast){27                     slow = slow->next;28                     fast = fast->next;29                 }30                 31                 return slow;32             }33         }34         return NULL;35     }36 };