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 || !head->next) return NULL;13         ListNode *slow = head, *fast = head;14         bool flag = false;15         while (fast && fast->next) {16             slow = slow->next;17             fast = fast->next->next;18             if (slow == fast) {19                 flag = true;20                 break;21             }22         }23         if (!flag) return NULL;24         fast = head;25         while (slow != fast) {26             slow = slow->next;27             fast = fast->next;28         }29         return slow;30     }31 };