141. Linked List Cycle（LeetCode）

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

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

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     bool hasCycle(ListNode *head) {
12         ListNode *slow = head, *fast = head;
13 
14     while ( fast && fast->next ) 
15     {
16         slow = slow->next;
17         fast = fast->next->next;
18         if ( slow == fast ) return true;
19     }
20 
21     return false;
22     }
23 };