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;

    }

};

LeetCode OJ - Linked List Cycle