Linked List Cycle --判断链表是否有环

问题：链接

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

Follow up:

Can you solve it without using extra space?
解答：
快慢指针。

代码：

/**
* 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 == NULL)
return false;
ListNode* slow = head;
ListNode* fast = head->next;
while(fast)
{
if(slow == fast)
return true;
slow = slow->next;
fast = fast->next;
if(fast == NULL)
break;
fast = fast->next;
}
return false;
}
};