#include <bits/stdc++.h>

using namespace std;

#define int long long
#define ff first
#define ss second
#define pb push_back

const int N = 2e5 + 5, inf = 1e18 + 100;

void solve()
{
    int n;
    cin >> n;

    string s;
    cin >> s;

    for (char c = 'a'; c <= 'z'; ++c)
    {
        int ok = 1;
        for (int i = 0; i < n; ++i)
            if (s[i] == c) {ok = 0; break;}
        if (ok) {cout << c << '\n'; return;}
    }

    for (char c = 'a'; c <= 'z'; ++c)
    {
        for (char c1 = 'a'; c1 <= 'z'; ++c1)
        {
            int ok = 1;
            for (int i = 0; i < n; ++i)
                if (i + 1 < n && s[i] == c && s[i + 1] == c1) {ok = 0; break;}
            if (ok) {cout << c << c1 << '\n'; return;}
        }
    }

    for (char c = 'a'; c <= 'z'; ++c)
    {
        for (char c1 = 'a'; c1 <= 'z'; ++c1)
        {
            for (char c2 = 'a'; c2 <= 'z'; ++c2)
            {
                int ok = 1;
                for (int i = 0; i < n; ++i)
                    if (i + 2 < n && s[i] == c && s[i + 1] == c1 && s[i + 2] == c2) {ok = 0; break;}
                if (ok) {cout << c << c1 << c2 << '\n'; return;}
            }
        }
    }
}

signed main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int q;
    cin >> q;
    while (q--)
        solve();
    return 0;
}
















