#include <bits/stdc++.h>
using namespace std;

#define ff first
#define ss second
#define int long long
#define ld long double
#define pb push_back
#define mt make_tuple
#define PI 3.14159265
#define pii pair<int, int>
#define vpii vector<pair<int, int>>
#define vi vector<int>
#define pqb priority_queue<int>
#define pqs priority_queue<int, vi, greater<int>>
#define mod 1000000007
#define all(v) (v).begin(), (v).end()
#define ps(x, y) fixed << setprecision(y) << x
#define setbits(x) __builtin_popcountll(x)
#define mk(arr, n, type) type *arr = new type[n];
#define dbg(x) cout << #x << "=" << (x) << endl;
#define dbg2(x, y) cout << #x << "=" << (x) << " " << #y << "=" << (y) << endl;
#define dbg3(x, y, z) cout << #x << "=" << (x) << " " << #y << "=" << (y) << " " << #z << "=" << (z) << endl;
#define dbg4(x, y, z, w) cout << #x << "=" << (x) << " " << #y << "=" << (y) << " " << #z << "=" << (z) << " " << #w << "=" << (w) << endl;
#define FIO                       \
    ios_base::sync_with_stdio(0); \
    cin.tie(0);                   \
    cout.tie(0)
#define endl "\n"
#define MOD 998244353
const int N = 1e5 + 1;
vi dp;

int32_t main()
{
    FIO;
    int t;
    t = 1;
    cin >> t;
    while (t--)
    {
        int n;
        cin >> n;
        string s;
        cin >> s;
        int yes = 1;
        string ans = "";
        for (char c = 'a'; c <= 'z' && yes; c++)
        {
            //cout<<to_string(c)<<endl;
            if (s.find(c) == string::npos)
            {

                ans = string(1,c);
                //cout << "HELLO1 " << ans << endl;
                yes = 0;
            }
        }
        for (char c1 = 'a'; c1 <= 'z' && yes; c1++)
        {
            for (char c2 = 'a'; c2 <= 'z' && yes; c2++)
            {
                string two = string(1,c1) + string(1,c2);
                //cout<<two<<endl;
                if (s.find(two) == string::npos)
                {
                    ans = two;
                    //cout << "HELLO2 " << ans << endl;
                    yes = 0;
                }
            }
        }
        for (char c1 = 'a'; c1 <= 'z' && yes; c1++)
        {
            for (char c2 = 'a'; c2 <= 'z' && yes; c2++)
            {
                for (char c3 = 'a'; c3 <= 'z' && yes; c3++)
                {
                    string three = string(1,c1) + string(1,c2) + string(1,c3);
                    if (s.find(three) == string::npos)
                    {
                        ans = three;
                        //cout << "HELLO3 " << ans << endl;
                        yes = 0;
                    }
                }
            }
        }
        cout << ans << endl;
    }
    cerr << "Time : " << ((double)clock()) / (double)CLOCKS_PER_SEC << "ms\n";
    return 0;
}