#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<queue>
using namespace std;
typedef long long int ll;
int T;
int n,tot,maxx;
char s[1000005];
bool vis[1000005];
int ans[10];
bool check(int now){
	while(now){
		if(now%27==0){
			return false;
		}
		now/=27;
	}
	return true;
}
void print(int now){
	while(now){
		ans[++tot]=now%27;
		now/=27;
	}
	for(int i=tot;i>=1;i--){
		cout<<char(ans[i]+'a'-1);
	}
	printf("\n");
}
int main()
{
//	freopen("B.out","r",stdin);
	scanf("%d",&T);
	while(T--){
		tot=0;maxx=0;
		scanf("%d",&n);
		scanf("%s",s+1);
		for(int i=1;i<=n;i++){
			vis[s[i]-'a'+1]=1;
			maxx=max(maxx,s[i]-'a'+1);
		}
		
		for(int i=1;i<n;i++){
			int sum=(s[i]-'a'+1)*27+(s[i+1]-'a'+1);
			vis[sum]=1;
			maxx=max(maxx,sum);
		}
		
		for(int i=1;i<n-1;i++){
			int sum=(s[i]-'a'+1)*27*27+(s[i+1]-'a'+1)*27+(s[i+2]-'a'+1);
			vis[sum]=1;
			maxx=max(maxx,sum);
		}
		
		for(int i=1;i<=maxx+1;i++){
			if(vis[i]==0&&check(i)){
				print(i);
				break;
			}
		}
		
		for(int i=1;i<=n;i++){
			vis[s[i]-'a'+1]=0;
		}
		
		for(int i=1;i<n;i++){
			int sum=(s[i]-'a'+1)*27+(s[i+1]-'a'+1);
			vis[sum]=0;
		}
		
		for(int i=1;i<n-1;i++){
			int sum=(s[i]-'a'+1)*27*27+(s[i+1]-'a'+1)*27+(s[i+2]-'a'+1);
			vis[sum]=0;
		}
	}
	return 0;
}


