#pragma GCC optimize("Ofast")
#include <iostream>
#include <fstream>
#include <stdio.h>
#include <string>
#include <vector>
#include <algorithm>
#include <cstdlib>
#include <cmath>
#include <iomanip>
#include <cctype>
#include <sstream>
#include <stack>
#include <deque>
#include <queue>
#include <list>
#include <set>
#include <map>
#include <chrono>
#include <random>
#include <unordered_map>  
//#include <atcoder/segtree>
//#include <atcoder/fenwicktree>

using namespace std;
//using namespace atcoder;
using ll = long long;
using P = pair<ll, ll>;
template <class T>using V = vector<T>;
template <class T>using VV = V<V<T>>;
template <class T, class U>bool chmin(T& t, const U& u) { if (t > u) { t = u; return 1; } else return 0; }
template <class T, class U>bool chmax(T& t, const U& u) { if (t < u) { t = u; return 1; } else return 0; }

#define REP(i,n) for(ll i=0;i<ll(n);i++)
#define FOR(i,a,b) for(ll i=ll(a);i<=ll(b);i++)
#define FORD(i,a,b) for(ll i=ll(a);i>=ll(b);i--)
#define MP make_pair
#define SZ(x) ll(x.size())
#define ALL(x) x.begin(),x.end()
#define INF 1000100010001000
#define CINF 1ll<<63-1
#define PI 3.14159265358979323846
#define endl "\n"
#define EPS 1e-9
#define YES cout << "YES" << endl
#define NO cout << "NO" << endl
#define Yes cout << "Yes" << endl
#define No cout << "No" << endl

void solve() {
    ll n; cin >> n;
    string s; cin >> s;
    map<string, ll> m;
    REP(i, n) {
        m[s.substr(i, 1)]++;
        if (i + 1 <= n - 1)m[s.substr(i, 2)]++;
        if (i + 2 <= n - 1)m[s.substr(i, 3)]++;
    }
    REP(i, 26) {
        string x = "";
        x += char('a' + i);
        if (m[x] == 0) {
            cout << x << endl;
            return;
        }
    }
    REP(i, 26) {
        string x = "";
        x += char('a' + i);
        REP(j, 26) {
            string y = x;
            y += char('a' + j);
            if (m[y] == 0) {
                cout << y << endl;
                return;
            }
        }
    }
    REP(i, 26) {
        string x = "";
        x += char('a' + i);
        REP(j, 26) {
            string y = x;
            y += char('a' + j);           
            REP(k, 26) {
                string z = y;
                z += char('a' + k);
                if (m[z] == 0) {
                    cout << z << endl;
                    return;
                }
            }
        }
    }


    return;
}

signed main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);
    
    ll t; cin >> t;
    while (t) {
        solve();
        t--;
    }
    
    
}