/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */ class Solution { public: bool hasCycle(ListNode *head) { if(head==NULL) { return head; } ListNode *slow = head; ListNode *fast = head; while(fast && fast->next != NULL) { fast = fast->next->next; slow = slow->next; if(fast == slow) { return true; } } return false; } };