#include <bits/stdc++.h>
#define mk make_pair
#define fs first
#define sc second
using namespace std;
typedef long long ll;
typedef long double ld;
// please, read the question correctly (do you need set or multiset)???
const int N=1010; //check the limits, dummy
int a[N];
int n, m;
char s[N];
bool check(string &c){
	int sz = c.size();
	for(int i=0; i+sz-1<n; ++i){
		bool l= 1;
		for(int j=i; j<i+sz; ++j){
			if(c[j-i]!=s[j]){
				l=0;
				break;
			}
		}
		if(l)return 1;
	}
	return 0;
}
int main(){
	vector<pair<int, string>> v;
	string s1, s2;
	for(int i=0; i<26; ++i){
		v.push_back(mk(1,string(1, char(i+'a'))));
		for(int j=0; j<26; ++j){
			s1=string(1, char(i+'a'))+string(1, char(j+'a'));
			v.push_back(mk(2,s1));
			for(int g=0; g<26; ++g){
				s2 = s1+string(1, char(g+'a'));
				v.push_back(mk(3,s2));
			}
		}
	}
	sort(v.begin(), v.end());
int t;cin>>t;while(t--){

	scanf("%d%s",&n,s);
	for(auto it:v){
		// cout<<it.sc<<endl;
		if(!check(it.sc)){
			cout<<it.sc<<endl;
			break;
		}
	}

}
return 0;
}