LeetCode OJ - Linked List Cycle

题目：

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

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

解题思路：

　　使用快慢指针，快指针每次走两步，慢指针每次走一步，若快指针能追上慢指针，则表明有圈。

代码如下：

/**
* 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 *quicker = head->next;
ListNode *slower = head;

while ((quicker != NULL && quicker->next != NULL) && slower != NULL && quicker != slower) {
quicker = quicker->next->next;
slower = slower->next;
}

return quicker == slower;
}
};