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