Linked List Cycle II

    ListNode *detectCycle(ListNode *head) {
        if(!head||!head->next)
            return NULL;
        ListNode *fast,*slow;
        fast=head;
        slow=head;
        while(fast&&fast->next)
        {
            slow=slow->next;
            fast=fast->next->next;
            if(fast==slow)
            {
                fast=head;
                while(fast!=slow)
                {
                    fast=fast->next;
                    slow=slow->next;
                }
                return fast;
            }
        }
        return NULL;
    }

AC