Linked List Cycle II

ListNode *detectCycle(ListNode *head) {
        if(!head||!head->next)return NULL;
        ListNode*slow=head,*fast=head;
        int flag=0;
        while(slow!=NULL&&fast!=NULL)
        {
            if(!fast||!fast->next)return NULL;
            slow=slow->next;
            fast=fast->next->next;
            if(slow==fast){flag=1; break;}
        }
        if(flag==0)return NULL;
        slow=head;
        while(slow!=fast)
        {
            slow=slow->next;
            fast=fast->next;
        }
        return slow;
    }