#include <bits/stdc++.h>

#define mx(a) *max_element(all(a))
#define mn(a) *min_element(all(a))
#define int long long int

#define pb push_back
#define mpp make_pair
#define F first
#define S second
#define vec vector<int>
#define pr vector<pair<int, int>>
#define vecv vector<vec>
#define vecs vector<string>
#define vecc vector<char>

#define pi 3.141592653589793238
#define mod 1000000007
#define Mod 998244353
#define inf 1e16
#define precise(i) cout << fixed << setprecision(i);
#define endl "\n"
#define yes cout << "YES" << endl;
#define no cout << "NO" << endl;
#define one cout << -1 << endl;

#define err1(a) cout << #a << " " << a << endl;
#define err2(a, b) cout << #a << " " << a << " " << #b << " " << b << endl;
#define err3(a, b, c) cout << #a << " " << a << " " << #b << " " << b << " " << #c << " " << c << endl;

#define fastio                        \
    ios_base::sync_with_stdio(false); \
    cin.tie(NULL);                    \
    cout.tie(NULL);
#define all(a) a.begin(), a.end()
#define show(a)             \
    for (auto xyz : a)      \
        cout << xyz << " "; \
    cout << endl;
#define show_nl(a)     \
    for (auto xyz : a) \
        cout << xyz << endl;
#define loop(a) for (auto xyz : a)
#define f(i, a, b) for (int i = a; i < b; i++)
#define rf(i, a, b) for (int i = b; i >= a; i--)

const long double eps = 1e-8;

#define lb lower_bound //THIS GIVES THE ITR TO THE ELEMENT IN RANGE [SI,EI) THAT IS >= OUR VALUE
#define ub upper_bound //THIS GIVES THE ITR TO THE ELEMENT IN RANGE [SI,EI) THAT IS > OUR VALUE
using namespace std;

int isSubstring(string s1, string s2)
{
    int M = s1.length();
    int N = s2.length();
    for (int i = 0; i <= N - M; i++)
    {
        int j;
        for (j = 0; j < M; j++)
            if (s2[i + j] != s1[j])
                break;

        if (j == M)
            return i;
    }

    return -1;
}

string nextWord(string s)
{
    int n = s.length();
    int i = n - 1;
    while (s[i] == 'z')
        i--;
    if (i == -1)
    {
        s = string(n + 1, 'a');
        return s;
    }
    s[i]++;
    i++;
    while (i < n)
    {
        s[i] = 'a';
        i++;
    }
    return s;
}

void solve()
{
    int n;
    cin >> n;
    string s;
    cin >> s;
    string t = "a";

    int k = 100;
    while (true)
    {
        if (isSubstring(t, s) != -1)
        {
            t = nextWord(t);
        }
        else
        {
            break;
        }
        //cout<<t<<endl;
    }
    cout << t << endl;
}

signed main()
{
    fastio;
    int t = 1;
    cin >> t;
    while (t--)
    {
        solve();
    }
}