/***
**      Bismillahir Rahmanir Rahim                
**              ALLAHU AKBAR
**
**     Author: Khairul Anam Mubin (__Broly__)
**     Bangladesh University of Business and Technology,
**     Dept. of CSE.
***/
#include <bits/stdc++.h>
using namespace std;
 
#define F            first
#define S            second  
#define Fin          freopen("input.txt","r",stdin)
#define Fout         freopen("output.txt","w",stdout)
#define Precision(a) cout << fixed << setprecision(a)
#define FasterIO     ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0); 
#define Test         int test;cin >> test;for(int tc = 1; tc <= test; tc++)
 
#define INF9         2147483647
#define INF18        9223372036854775806
#define eps          1e-8 
 
const double pi = 2 * acos(0.0);
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
 
template <typename T> T Sqr(T x) { T n = x * x ; return n ;}
template <typename T> T Pow(T B,T P){ if(P==0) return 1; if(P&1) return B*Pow(B,P-1);  else return Sqr(Pow(B,P/2));}
template <typename T> T Abs(T a) {if(a<0)return -a;else return a;}
template <typename T> T Gcd(T a,T b){if(a<0)return Gcd(-a,b);if(b<0)return Gcd(a,-b);return (b==0)?a:Gcd(b,a%b);}
template <typename T> T Lcm(T a,T b) {if(a<0)return Lcm(-a,b);if(b<0)return Lcm(a,-b);return a*(b/Gcd(a,b));}
template <typename T> T Exgcd(T a,T b,T &x,T &y) {if(a<0){T d=Exgcd(-a,b,x,y);x=-x;return d;}   if(b<0){T d=Exgcd(a,-b,x,y);y=-y;return d;}   if(b==0){x=1;y=0;return a;}else{T d=Exgcd(b,a%b,x,y);T t=x;x=y;y=t-(a/b)*y;return d;}}
template <typename T> T BigMod (T b,T p,T m){if (p == 0) return 1;if (p%2 == 0){T s = BigMod(b,p/2,m);return ((s%m)*(s%m))%m;}return ((b%m)*(BigMod(b,p-1,m)%m))%m;}
template <typename T> T ModInvPrime (T b,T m){return BigMod(b,m-2,m);}
template <typename T> T ModInvNotPrime(T a , T m) {T x , y ;Exgcd(a , m , x , y) ;x %= m ;if(x < 0) x += m ;return x ;}
template <typename T> typename std::vector<T>::iterator Insert_sorted (std :: vector<T> & vec, T const& item) {return vec.insert (std::upper_bound( vec.begin(), vec.end(), item ), item);}
template <typename T> inline string ToBinary(T n) {string r ;while(n != 0) {r = ( n % 2 == 0 ? "0" : "1" ) + r ; n >>= 1;} return r ;} 
long long BinaryToDecimal(string s) {int len = s.size();long long n = 0, p = 1;for (int i = len - 1; i >= 0; i-- , p *= 2) n += p * (s[i] - '0');return n;}
 
char Uplowch(char ch){if(ch >= 'A' &&  ch <= 'Z') ch += 32; return ch;}
char Lowupch(char ch){if(ch >= 'a' &&  ch <= 'z') ch -= 32; return ch;}
bool Isalpha(char ch){if((ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z')) return true; return false;}
int Strtoint(string str){stringstream ss(str);int x = 0;ss >> x ;return x ;}
string Intostr(int x){stringstream ss; ss << x; string str = ss.str(); return str;}
vector<string> Linetostr(string str){string s; vector <string> v ;istringstream is(str); while(is >> s)v.push_back(s);return v;}
template <typename T> void Print(T ar[] , int a , int b) {for (int i = a; i + 1 <= b ; i++)cout << ar[i] << " ";cout << ar[b] << "\n";}
template <typename T> void Print(T ar[] , int n) {for (int i = 0; i + 1 < n; i++)cout << ar[i] << " ";cout << ar[n - 1] << "\n";} 
template <typename T> void Print(const vector<T> &v) {for (int i = 0; i + 1 < v.size() ; i++) cout << v[i] << " ";cout << v.back() << "\n";}
 
//int dx[] = {0 , 0 , -1 , 1 , -1 , -1 , 1 , 1 , 0} ;  // right , left , up , down
//int dy[] = {1 ,-1 , 0 , 0 , -1 , 1 , -1 , 1 , 0} ;

string Next(string a) {
    int sz = a.size();
    if (a.back() == 'z') {
        bool flag = 0;
        a[sz - 1] = 'a';
        for (int i = sz - 2; i >= 0; i--) {
            if (a[i] != 'z') {
                a[i]++;
                flag = 1;
                break;
            }
        }
        if (flag) return a;
        a.clear();
        for (int i = 0; i <= sz; i++) a.push_back('a');
        return a;
    } else {
        a[sz - 1]++;
    }
    return a;
}
int main() {
    FasterIO
    Test {
        int n; cin >> n;
        string s; cin >> s;
        ll here = 26;
        string ans = "";
        for (int i = 1; i <= n; i++) {
            set <string> st;
            for (int j = 0; j + i <= n; j++) {
                st.insert(s.substr(j , i));
            }
            int len = i;
            vector <string> v;
            for (string it : st) {
                v.push_back((it));
            }
            string tmp = "";
            for (int j = 0; j < len; j++) {
                tmp.push_back('a');
            }
            if (v.size() != here) {
                for (int j = 0; j < v.size(); j++) {
                    if (tmp != v[j]) {
                        ans = tmp;
                        break;
                    } 
                    tmp = Next(tmp);       
                }
                ans = tmp; 
            }
            if (ans != "") break;
            here *= 26; 
        }
        if (ans == "") {
            ans = Next(s);
        }
        cout << ans << "\n";
    }
    return 0;
}