判断链表是否有环 Linked List Cycle

题目源自于leetcode。

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

Follow up:

Can you solve it without using extra space?

思路：快指针、 慢指针。有环的话，一定不会遇到NULL；没环的话，fast一定先到NULL。该算法的时间复杂度为O（N），空间复杂度为O（1）。

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