#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <cstdio>
#include <cstdlib>
#include <queue>
#include <list>
#include <cmath>
#include <string>
#include <cstring>
typedef long long LL;
using namespace std;

string zz(int n)
{
    string s = "";
    while(n > 0){
		int m = n % 26;
		if(m == 0) m = 26;
		s += (char)(m + 97 - 1);
		n = (n - m) / 26;
	}
	reverse(s.begin(), s.end());	//反序
    return s;
}

int main()
{
    int t;
    cin >> t;
    while (t --)
    {
        int n;
        cin >> n;
        string s;
        cin >> s;

        
        for (int i = 1; ; i++)
        {
            if (s.find(zz(i)) == s.npos) 
            {
                cout << zz(i) << endl;
                break;
            } 
        }  
        
    }
    // system("pause");
    return 0;
}
 		 	 			 		   	  		    		  	 	