#include<bits/stdc++.h>
using namespace std;
 
#define ll		long long
#define ppb     pop_back
#define pb 		push_back
#define mk      make_pair
#define ff      first
#define ss      second
#define endl	"\n"
#define fast	std::ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);	
#define MOD     1000000007

void solve()
{
	int n;
	cin >> n;
	string str;
	cin >> str;
	map<char,int>mp;
	for(int i = 0; i < str.size(); i++)
	{	
		mp[str[i]]++;
	}
	for(int i = 0; i < 26; i++)
	{	
		// cout << 'a' + i << " ";
		if(mp['a' + i] == 0)
		{
			cout << char('a' + i) << endl;
			return;
		}
	}
	map<string,int>mp1;
	for(int i = 0; i < str.size()-1; i++)
	{	
		string temp = "";
		temp += str[i];
		mp1[temp + str[i+1]]++;
	}
	for(int i = 0; i < 26; i++)
	{	
		string temp1 = "";
		temp1 += ('a' + i);
		for(int j = 0; j < 26; j++)
		{	
			if(mp1[temp1 + char('a' + j)] == 0)
			{
				cout << temp1 + char('a' + j) << endl;
				return;
			}
		}
	}
	map<string,int>mp2;
	for(int i = 0; i < str.size()-2; i++)
	{	
		string temp = "";
		temp += str[i];
		mp2[temp + str[i+1] + str[i+2]]++;
	}
	// cout << mp1.size() << endl;
	
	for(int i = 0; i < 26; i++)
	{	
		string temp1 = "";
		temp1 += ('a' + i);
		for(int j = 0; j < 26; j++)
		{	
			for(int k = 0; k < 26; k++)
			{
				if(mp2[temp1 + char('a' + j) + char('a' + k)] == 0)
				{
					cout << temp1 + char('a' + j) + char('a' + k)<< endl;
					return;
				}
			}
			
		}
	}
	return;
	
}

signed main()
{
	fast;
	int t=1;
	cin>>t;
	while(t--)
	{
		solve();
	}
	
}