/** * 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 || !head -> next) return false;        ListNode *l1 = head, *l2 = head -> next;        while(l1 && l2)        {            if (l1 == l2)                return true;            l1 = l1 -> next;             l2 = l2 -> next;            if (!l2)                break;            l2 = l2 -> next;        }        return false;    }};