leetcode[141]Linked List Cycle

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

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

/**

 * 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;

        while(fast&&fast->next)

        {

            fast=fast->next->next;

            slow=slow->next;

            if(fast==slow)return true;

        }

        return false;

    }

};