#include<bits/stdc++.h>
#define pb push_back
#define int long long
#define f first
#define s second
using namespace std;
int MOD = 1e9 + 7 ;

int add(int a, int b)
{
    a += b;
    if (a >= MOD) a -= MOD;
    if (a < 0) a += MOD;
    return a;
}

int mul(int a, int b)
{
    return a * 1ll * b % MOD;
}

bool findsub ( string &s , string sub )
{
    if ( s . find ( sub ) == string :: npos )
        {
            return 1;
        }
    return 0;
}

void solve ()
{
    int n;
    string s;
    cin >> n >> s;
    for ( char i = 'a' ; i <= 'z' ; i++ )
    {
        string sub = "";
        sub += i;
        if ( findsub ( s , sub ) )
        {
            cout << sub << endl;
            return;
        }
    }
    for ( char i = 'a' ; i <= 'z' ; i++ )
    {

        for ( char j = 'a' ; j <= 'z' ; j++ )
        {
            string sub = "";
            sub += i;
            sub += j;
            if ( findsub ( s , sub ) )
            {
                cout << sub << endl;
                return;
            }
        }
    }
    for ( int  k = 'a' ; k <= 'z' ; k++ )
    for ( char i = 'a' ; i <= 'z' ; i++ )
    {

        for ( char j = 'a' ; j <= 'z' ; j++ )
        {
            string sub = "";
            sub += k;
            sub += i;
            sub += j;

            if ( findsub ( s , sub ) )
            {
                cout << sub << endl;
                return;
            }
        }
    }
    for ( char l = 'a' ; l <= 'z' ; l++ )
    for ( char  k = 'a' ; k <= 'z' ; k++ )
    for ( char i = 'a' ; i <= 'z' ; i++ )
    {

        for ( char j = 'a' ; j <= 'z' ; j++ )
        {
            string sub = "";
            sub += l;
            sub += k;
            sub += i;
            sub += j;

            if ( findsub ( s , sub ) )
            {
                cout << sub << endl;
                return;
            }
        }
    }

}


int32_t main()
{
        ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);

        int t = 1;
        cin >> t;

        while ( t-- )
            solve ();

        return 0;
}

