Wrong Answer on test #2（几天不见你又来了）

#include<bits/stdc++.h>
using namespace std;
string s;
int main(){
	int n,st,en;int ans;char a2;
	cin>>n;
	st=1;
	while(st<n){
		for(en=st+1;en<=n;en++){
			cout<<"? 2 "<<st<<" "<<en<<endl;
			cin>>ans;
			if(ans>1){
				cout<<"? 1 "<<st<<endl;
				cin>>a2;
				for(int i=st;i<en;i++) s+=a2;
				break;
			}
		}
		if(en==n+1){
			cout<<"? 1 "<<st<<endl;cin>>a2;
			for(int i=st;i<=n;i++) s+=a2;
			break;
		}
		st=en;
	}
	cout<<"! "<<s<<endl;
	return 0;
}