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) {
ListNode *slow=head,*fast=head;
if(!head)
return 0;
if(fast->next)
fast=fast->next->next;
else
return 0;
slow=slow->next;
while(fast && fast->next)
{
if(fast->val==slow->val)
return true;
else
{
fast=fast->next->next;
slow=slow->next;
}
}

return false;
}
};

Runtime: 76 ms