Leetcode 141. Linked List Cycle

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

Follow up:
Can you solve it without using extra space?

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