#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;


int main(){
    ll t; cin>> t;
    while(t--){
        ll n; cin>> n;
        bool gta[2000]={false};
        string s, y1, y2, y3; cin>> s;
        for(ll i=0; i<n; i++){
            ll r[3]={0};
            for(ll j=i; j<i+3 && j<n; j++){
                char f=s[j];
                ll k=(f-'a');
                r[j-i]=k;
            }
            gta[r[0]]=1;
            if(i+1<n){
                gta[r[0]*26+r[1]+26]=1;
            }
            if(i+2<n && r[0]==0){
                gta[r[1]*26+r[2]+702]=1;
            }
        }
        ll u=0;
        while(gta[u]==1) u++;
        if(u<26){
            char fd=('a'+u); cout<< fd<< endl;
        }
        else if(u<702){
            u-=26;
            char fd=('a'+(u/26)), fr=('a'+(u%26)); cout<< fd<< fr<< endl;
        }
        else{
            u-=702;
            char fd=('a'+(u/26)), fr=('a'+(u%26)); cout<< 'a'<< fd<< fr<< endl;
        }
    }
}
