Linked List Cycle

Description:

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

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

Code:

bool hasCycle(ListNode *head) {

if (head == NULL)
return false;
ListNode* p = head;
ListNode* q = head;

while (p&&q)
{
p = p->next;
q = q->next;
if (p==NULL||q==NULL)
return false ;
q = q->next;
if (p==q)
return true;
}
return false;
}