#include <algorithm>
#include <cmath>
#include <iostream>
#include <iomanip>
#include <map>
#include <set>
#include <string>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <unordered_map>
#include <numeric>
#include <random>

using namespace std;
//struct bor {
//    int ch[26];
//};
//vector<bor> b;
signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    int t;
    cin >> t;
    while (t--) {
        /* b.clear();
         b.push_back(bor());*/
        int n;
        string s;
        cin >> n >> s;
        /*for (int i = 0; i < n; ++i) {
            add(s.substr(i, n - i));
        }
        pr(string)*/
        bool ok = 0;
        for (char i = 'a'; i <= 'z'; ++i) {
            if (s.find(i) == -1) {
                cout << i << '\n';
                ok = 1;
                break;
            }
        }
        if (!ok) {
            int kek = pow(26, 2);
            for (int i = 0; i < kek; ++i) {
                string t(2, 'a');
                t[0] += i / 26;
                t[1] += i % 26;
                if (s.find(t) == -1) {
                    cout << t << '\n';
                    ok = 1;
                    break;
                }
            }
        }
        if (!ok) {
            int lol = pow(26, 3);
            int kek = pow(26, 2);
            for (int i = 0; i < lol; ++i) {
                string t(3, 'a');
                t[0] += i / kek;
                t[1] += i / 26;
                t[2] += i % 26;
                if (s.find(t) == -1) {
                    cout << t << '\n';
                    ok = 1;
                    break;
                }
            }
        }
    }
    return 0;
}
