#include <bits/stdc++.h>
using namespace std;

#define int int64_t

#define fast_io() ios_base::sync_with_stdio(false),cin.tie(nullptr);

void solve(){
    int n; cin >> n;
    string s; cin >> s;
    int freq[26] = {0};
    for(int i = 0; i < (int)s.size() ; ++i ){
        freq[s[i]-'a'] = 1;
    }
    for(int i = 0; i < 26; ++i ){
        if(freq[i] == 0){
            cout << char('a' + i) << '\n';
            return;
        }
    }
    // for the lenght of the 2
    set<string>s1;
    for(int i = 0; i < (int)s.size()-1; ++i ){
        string temp = "";
        temp += s[i];temp+= s[i+1];
        s1.insert(temp);
    }
    for(int i = 0 ; i < 26; ++i){
        for(int k = 0 ; k < 26; ++k){
            string f = "";
            f += (char)('a' + i);
            f += (char)('a' + k);
            if(s1.count(f) == 0){
                cout << f << '\n';
                return;
            }
        }
    }

    // for the lenght of the 3
    set<string>x;
    for(int i = 0 ; i < (int)s.size()-2;++i){
        string temp = "";
        temp += s[i];
        temp += s[i+1];
        temp += s[i+2];
        x.insert(temp);
    }
    for(int i = 0; i < 26;i++)
        for(int o = 0; o < 26; ++o){
            string temp = "a";
            temp += (char)('a' + i);
            temp += (char)('a' + o);
            if(x.count(temp) == 0){
                cout << temp << "\n";return;
            }
        }
}

int32_t main(){
    int t ; cin >> t;
    while (t--){
        solve();
    }
    return 0;
}
