[leetcode]Linked List Cycle

http://oj.leetcode.com/problems/linked-list-cycle/

老题，快慢指针。

/**

 * 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 *fast = head;

        ListNode *slow = head;

        do {

            if (fast == NULL || fast->next == NULL) return false;

            fast = fast->next->next;

            slow = slow->next;

        } while (fast != slow);

        return true;

    }

};