#ifdef LOCAL
#define N 100
#define K 5
#else
#define N 1005
#define K 18
#endif

#include <bits/stdc++.h>

using namespace std;
typedef long long int lld;


char chaine[N];
string s;

bool gen(int pos, int nb){
    if(pos == nb){
        for(int i = 0; i + nb - 1 < s.size(); i++){
            bool match = true;
            for(int j = 0; j < nb; j++){
                if(s[i+j] != chaine[j]){
                    match = false;
                    break;
                }
            }
            if(match){
                return false;
            }
        }
        return true;
    }
    for(int ch = 0; ch < 26; ch++){
        chaine[pos] = 'a' + ch;
        if(gen(pos+1, nb)){
            return true;
        }
    }
    return false;
}

void prob()
{
    int n;
    cin >> n;
    cin >> s;
    fill_n(chaine, n+1, 0);
    int t = 1;
    while(!gen(0, t)){
        t++;
    }
    cout << chaine << '\n';
}

int main()
{
#ifdef LOCAL
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int t;
    cin >> t;
    for (int i = 0; i < t; i++)
    {
        prob();
    }

    return 0;
}