#include <bits/stdc++.h>
#define M 1000000007
#define fio                           \
    ios_base::sync_with_stdio(false); \
    cin.tie(NULL);
#define d(x) cout << #x << " " << x << "\n";
#define min(x1, x2) (x1 > x2 ? x2 : x1)
#define max(x1, x2) (x1 < x2 ? x2 : x1)
#define min3(x1, x2, x3) (x3 > min(x1, x2) ? min(x2, x1) : x3)
#define max3(x1, x2, x3) (x3 < max(x1, x2) ? max(x1, x2) : x3)
#define ll long long int
#define ul unsigned long long int
#define ld long double
#define dv(v)                            \
    cerr << #v << "  ";                  \
    for (int i = 0; i < (v).size(); i++) \
        cerr << v[i] << " ";             \
    cerr << "\n";
#define inf INT_MAX
#define mp(x, y) make_pair(x, y)
using namespace std;

bool rabin_karp(string const &s, string const &t)
{
    const int p = 31;
    const int m = 1e9 + 9;
    int S = s.size(), T = t.size();

    vector<long long> p_pow(max(S, T));
    p_pow[0] = 1;
    for (int i = 1; i < (int)p_pow.size(); i++)
        p_pow[i] = (p_pow[i - 1] * p) % m;

    vector<long long> h(T + 1, 0);
    for (int i = 0; i < T; i++)
        h[i + 1] = (h[i] + (t[i] - 'a' + 1) * p_pow[i]) % m;
    long long h_s = 0;
    for (int i = 0; i < S; i++)
        h_s = (h_s + (s[i] - 'a' + 1) * p_pow[i]) % m;

    for (int i = 0; i + S - 1 < T; i++)
    {
        long long cur_h = (h[i + S] + m - h[i]) % m;
        if (cur_h == h_s * p_pow[i] % m)
        {
            return 1;
        }
    }
    return 0;
}

int main()
{
    fio;
    ll t, n, x, y, z, k;
    cin >> t;
    while (t--)
    {
        string s;
        cin >> n >> s;
        char c = 'a';
        string s0;
        bool b0 = 0;
        string o;
        vector<string> s1;
        s1.push_back("");
        for (int l = 0; l < 3; ++l)
        {
            vector<string> s2;
            for (int j = 0; j < s1.size(); ++j)
            {
                for (int i = 0; i < 26; ++i)
                {
                    s1[j].push_back('a' + i);
                    if (!rabin_karp(s1[j], s))
                    {
                        cout << s1[j] << "\n";
                        b0 = 1;
                        break;
                    }
                    s2.push_back(s1[j]);
                    s1[j].pop_back();
                }
                if (b0)
                {
                    break;
                }
            }
            if (b0)
            {
                break;
            }
            s1 = s2;
        }
    }
    return 0;
}