#include<bits/stdc++.h>
using namespace std;
#define INIT  ios_base :: sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define mp make_pair
#define pb push_back
#define ft first
#define sc second
#define ll long long
#define pii pair<int, int>
#define count_bits __builtin_popcount
#define int ll

int t, n, m, k, a[300010], q, l, r;
char s[100010];

map<string, bool> h;


int32_t main(){
INIT

cin>>t;

while(t--){
    cin>>n;
    for(int i=1; i<=n; i++){
        cin>>s[i];
    }
    for(int l=1; l<=3; l++){
        for(int i=l; i<=n; i++){
            string s0="";
            for(int j=i-l+1; j<=i; j++){
                s0.pb(s[j]);
            }
            h[s0]=true;
        }
    }

    for(char a=('a'-1); a<='z'; a++){
        for(char b=('a'-1); b<='z'; b++){
            for(char c=('a'); c<='z'; c++){
                string s0="";
                if(a>='a' ){
                    s0.pb(a);
                }
                if( (b>='a') ){
                    s0.pb(b);
                }
                s0.pb(c);
                if(h[s0]==false){
                    cout<<s0<<"\n";
                    goto Next;
                }
            }
        }
    }
    Next:;
    h.clear();
}

return 0;
}
