/** * 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) {        ListNode* slow = head;        ListNode* fast = head;        while(fast && fast->next){            slow = slow->next;            fast = fast->next->next;            if(slow == fast) return true;        }        return false;    }};