Linked List Cycle

Given a linked list, determine if it has a cycle in it.
LintCode上原题

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