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?

最初代码，错误内容已写明：

bool solution( ListNode *head )
{
	ListNode *one = head, *second = head;
	
	//就这一行而言，对于{1->NULL} 或{}应为no cycle，导致出错 
	if( head == NULL || head->next == NULL )
		return true;
	
	while( one && second )
	{
		//对于{1->2->NULL} 这样的，应为no cycle，
		//但one和second先呗判断才赋值，导致死循环。
		//为了保证second->next->next不非法访问，只需确保second->next不为NULL即可。 
		if( one->next )
			one = one->next;
		if( second->next->next )
			second = second->next->next;
		if( one == second )
			return true;			
	}
	
	return false;
}

bool hasCycle(ListNode *head) {
       ListNode *one = head, *second = head;
        
        while( one && second && second->next )
        {
            one = one->next;
            second = second->next->next;
                
            if( one == second )
                return true;
        }
        
        return false;
    }