//ceil(a/b) is equal to (a+b-1)/b
#include<bits/stdc++.h>
 
#define mx(a) *max_element(all(a))
#define mn(a) *min_element(all(a))
#define int long long int
 
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define vec vector <int>
#define pr vector<pair<int,int>> 
#define vecv vector <vec>
#define vecs vector <string>
#define vecc vector <char>
 
#define pi 3.141592653589793238
#define mod 1000000007
#define Mod 998244353
#define inf 1e16
#define precise(i) fixed << setprecision(10) << i
#define endl "\n"
#define yes cout<<"YES"<<endl;
#define no cout<<"NO"<<endl;
#define one cout<<-1<<endl;

#define err1(a) cout<<#a<<" "<<a<<endl;
#define err2(a,b) cout<<#a<<" "<<a<<" "<<#b<<" "<<b<<endl;
#define err3(a,b,c) cout<<#a<<" "<<a<<" "<<#b<<" "<<b<<" "<<#c<<" "<<c<<endl;
 
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL); 
#define all(a) a.begin(),a.end()
#define show(a) for(auto xyz:a)cout<<xyz<<" ";cout<<endl;
#define show_nl(a) for(auto xyz:a)cout<<xyz<<endl;
#define loop(a) for(auto xyz:a)
#define f(i,a,b) for(int i=a;i<b;i++)
#define rf(i,a,b) for(int i=b;i>=a;i--)

const long double eps = 1e-8;

#define lb lower_bound  //THIS GIVES THE ITR TO THE ELEMENT IN RANGE [SI,EI) THAT IS >= OUR VALUE 
#define ub upper_bound  //THIS GIVES THE ITR TO THE ELEMENT IN RANGE [SI,EI) THAT IS > OUR VALUE 
 
using namespace std;
 
signed main() {
     fastio
     int t=1;
     cin>>t;
     while(t--)
     {
         int n;
        cin>>n;
        string s;
        cin>>s;
         vec fre(26);
         f(i,0,n)
         {
             fre[s[i]-'a']++;
         }
         string ans="";
         f(i,0,26)
         {
             if(fre[i]==0)
             {
               ans+=('a'+i);  
               break;
             }
         }
         if(ans.empty())
         {
             set <string> s1;
             f(i,0,n-1)
             {
                 string temp;
                 temp.pb(s[i]);
                 temp.pb(s[i+1]);
                 s1.insert(temp);
             }
             int f1=0;
             f(i,0,26)
             {
                 f(j,0,26)
                 {
                     string tt;
                     tt.pb('a'+i);
                     tt.pb('a'+j);
                     if(s1.find(tt)==s1.end())
                     {
                         ans=tt;
                         f1=1;
                         break;
                     }
                 }
                 if(f1) break;
             }
             if(f1)
             {
                 cout<<ans<<endl;
             }
              else
                 {
                      set <string> ss;
             f(i,0,n-2)
             {
                 string temp;
                 temp.pb(s[i]);
                 temp.pb(s[i+1]);
                 temp.pb(s[i+2]);
                 ss.insert(temp);
             }
             int f2=0;
             f(i,0,26)
             {
                 f(j,0,26)
                 {
                     f(k,0,26)
                     {
                     string tt;
                     tt.pb('a'+i);
                     tt.pb('a'+j);
                     tt.pb('a'+k);
                     if(ss.find(tt)==ss.end())
                     {
                         ans=tt;
                         f2=1;
                         break;
                     }
                     }
                     if(f2) break;
                 }
                 if(f2) break;
             }
                 cout<<ans<<endl;
             }
         }
         else
         {
             cout<<ans<<endl;
         }
     }
}