字符串做法60分求助

#include<bits/stdc++.h>
using namespace std;
queue<string> book;
int n,q;
string minn;
string minx(string x,string y) {
	if (x.size() < y.size()) return x;
	if (y.size() < x.size()) return y;
	for(int i=0;i<x.size();i++){
		if(x[i]>y[i]) {return y;break;}
		else if(y[i]>x[i]) {return x;break;}
	}
	return x;
}

int main(){
	cin>>n>>q;
	for(int i=1;i<=n;i++){
		string l;cin>>l;
		book.push(l);
	}
	for(int i=1;i<=q;i++){
		int x;cin>>x;
		string z;cin>>z;
		bool flag=false;
		for(int j=1;j<=book.size();j++){
			string y=book.front(),tmp=y;
			book.pop();	
			if(y.size()<z.size()) continue;
			tmp.erase(0,y.size()-x);
			if(tmp==z){
				flag=true;
				if(minn.empty()) minn=y;
				else{
					minn=minx(minn,y);
				}
			}
			book.push(y);
		}
		if(flag) cout<<minn<<endl;
		else cout<<"-1"<<endl;
		flag=false;
		minn.erase(0,minn.size());
	}
	return 0;
}