class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode*L=head,*R=head;
        bool hadCircle=false;
        while(R&&R->next){
            L=L->next;
            R=R->next->next;
            if(L==R){
                hadCircle=true;
                break;
            }
        }
        if(hadCircle){
            ListNode*ans=head;
            while(ans!=L)ans=ans->next,L=L->next;
            return ans;
        }
        return nullptr;
    }
};