LeetCode_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 *pSingleSpeed=head;         ListNode *pDpubleSpeed=head;         while (pSingleSpeed!=NULL&&pDpubleSpeed!=NULL)         {             pSingleSpeed=pSingleSpeed->next;             pDpubleSpeed=pDpubleSpeed->next;             if (pDpubleSpeed!=NULL)             {                 pDpubleSpeed=pDpubleSpeed->next;             }             else             {                 return false;             }             if (pSingleSpeed==pDpubleSpeed&&pSingleSpeed!=NULL                 &&pDpubleSpeed!=NULL)             {                 return true;             }         }         return false;        }};