#include<bits/stdc++.h>
using namespace std;
#define ll long long int

int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	
	int t;
	cin >> t;
	vector <pair<int,string>> b;
	string x;
	x = "abcdefghijklmnopqrstuvwxyz";
	int count = 0;
	for(int i=0;i<x.size();i++){
		string r;
		r = x[i];
		b.push_back(make_pair(1,r));
		count++;
	}	
	for(int i=0;i<x.size();i++){
		for(int j=0;j<x.size();j++){
			string r;
			r+=x[i];
			r+=x[j];
			b.push_back(make_pair(2,r));
			count++;
		}
	}
	int flag=0;
	for(int i=0;i<x.size();i++){
		for(int j=0;j<x.size();j++){
			for(int k=0;k<x.size();k++){
				string r;
				r+=x[i];
				r+=x[j];
				r+=x[k];
				b.push_back(make_pair(3,r));
				count++;
				if(count>1050){
					flag++;
					break;
				}
			}
			if(flag>0){
				break;
			}
		}
		if(flag>0){
			break;
		}
	}
	sort(b.begin(),b.end());
	while(t--){
		int n;
		cin >> n;
		string s;
		cin >> s;
		set <pair<int,string>> y;
		vector <pair<int,string>> a;
		for(int i=0;i<n;i++){
			string q;
			for(int j=i;j<min(n,i+3);j++){
				q+=s[j];
				y.insert(make_pair(q.size(),q));
			}
		}
		for(auto it:y){
			a.push_back(it);
		}
		for(int i=0;i<b.size();i++){
			if(b[i]!=a[i]){
				//cout << a[i].first << " " << a[i].second << " " << b[i].first << " " << b[i].second << endl;
				cout << b[i].second << endl;
				break;
			}
		}
	}
}