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;
}
};