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

#ifdef FFS
#include "debug.h"
#else
#define debug(args...)
#endif

void solve() {
    int n; cin >> n;
    string s; cin >> s;
    for(int i = 0; i < 26; ++i) {
        string now;
        now.push_back((char) ('a' + i));
        if((int) s.find(now) == -1 ) {
            cout << now << '\n';
            return;
        }
    }
    for(int i = 0; i < 26; ++i) {
        for(int j = 0; j < 26; ++j) {
            string now;
            now.push_back((char) ('a' + i));
            now.push_back((char) ('a' + j));
            if((int) s.find(now) == -1 ) {
                cout << now << '\n';
                return;
            }
            
        }
    }
    for(int i = 0; i < 26; ++i) {
        for(int j = 0; j < 26; ++j) {
            for(int k = 0; k < 26; ++k) {
                string now;
                now.push_back((char) ('a' + i));
                now.push_back((char) ('a' + j));
                now.push_back((char) ('a' + k));
                if((int) s.find(now) == -1) {
                    cout << now << '\n';
                    return;
                }
            }
        }
    }

}

signed main() {
    ios_base::sync_with_stdio(false), cin.tie(nullptr);
    int ts; cin >> ts;
    for (int cs = 1; cs <= ts; cs++) {
        //cout << "Case " << cs << ": ";
        solve();
    }
    return 0;
}