/******************************************************************************

                              Online C++ Compiler.
               Code, Compile, Run and Debug C++ program online.
Write your code in this editor and press "Run" button to compile and execute it.

*******************************************************************************/

#include<bits/stdc++.h>
using namespace std;
#define ll long long
 void test(){
 int n;cin>>n;
string p;cin>>p;

 
//vector<string>s;
 
for(char c='a';c<='z';c++){
        string ss="";
ss+=c;
if(p.find(ss)==string::npos){
    cout<<ss<<endl;
    return;
}
 
}
 
for(char c='a';c<='z';c++){
        for(char cc='a';cc<='z';cc++){
         string pp="";
        pp+=c;
        pp+=cc;
        if(p.find(pp)==string::npos){
          cout<<pp<<endl;
          return;
        }
}
}
 
for(char c='a';c<='z';c++){
        for(char cc='a';cc<='z';cc++){
                for(char cd='a';cd<='z';cd++){
                  string ss="";
                  ss+=c;
                  ss+=cc;
                  ss+=cd;
    if(p.find(ss)==string::npos){
    cout<<ss<<endl;
    return;
    }
}}
}
 
}
int main(){
ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int t;cin>>t;
	while(t--){
            test();
  
 
}
 
return 0;}
