#include<bits/stdc++.h>
using namespace std;
#define mod        1000000007
#define pb         push_back
#define mp         make_pair
#define rep(n)     for(int i=0;i<n;i++)
#define repit(x)   for(auto it : x)
#define ll         long long
#define vi         vector<int>
#define vc         vector<char>
#define vs         vector<string>
#define mii        map<int ,int>
#define mci        map<char ,int>
/* bool prime[100001];
 const int N=100000;
 void sieve() {
 int i,j;
for(i=2;i<=N;i++)
prime[i]=true;
 prime[0]=prime[1]=false;
for(i=2;i*i<=N;i++){
if(prime[i]){
for(j=i*i;j<=N;j+=i)
prime[j]=false;
}
}
}*/
int gcd(int a,int b){
if(b==0)
return a;
else
return gcd(b,a%b);
}
void swap(int *x,int *y){
int temp=*x;
*x=*y;
*y=temp;
}
int main()
{
    
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t;
cin>>t;
while(t--){
int i,n,c=0,j;
cin>>n;
string s,s2;
cin>>s;
map<string,int>m;
for(i=97;i<=122;i++){
    string s1;
    s1.push_back(i);
    m.insert(make_pair(s1,0));
}
for(i=0;i<n;i++){
    string s1;
    s1.push_back(s[i]);
    m[s1]++;
}
for(auto it=m.begin();it!=m.end();it++){
    if(it->second==0){
        c=1;
        s2=it->first;
        break;
    }
}
if(c){
    cout<<s2<<"\n";
}
else{
    m.clear();
    for(i=97;i<=122;i++){
        for(j=97;j<=122;j++){
            string s1;
            s1.push_back(i);
            s1.push_back(j);
            m.insert(make_pair(s1,0));
        }
    }
    for(i=0;i<n-1;i++){
        string s1;
        s1.push_back(s[i]);
        s1.push_back(s[i+1]);
        m[s1]++;
    }
    for(auto it=m.begin();it!=m.end();it++){
        if(it->second==0){
            c=1;
            s2=it->first;
            break;
        }
    }
    if(c)
    cout<<s2<<"\n";
    else{
        m.clear();
        for(i=97;i<=122;i++){
            for(j=97;j<=122;j++){
                for(int k=97;k<=122;k++){
                    string s1;
                    s1.push_back(i);
                    s1.push_back(j);
                    s1.push_back(k);
                    m.insert(make_pair(s1,0));
                }
            }
        }
        for(i=0;i<n-2;i++){
            string s1;
            s1.push_back(s[i]);
            s1.push_back(s[i+1]);
            s1.push_back(s[i+2]);
            m[s1]++;
            
        }
        for(auto it=m.begin();it!=m.end();it++){
            if(it->second==0){
                c=1;
                s2=it->first;
                break;
            }
        }
        cout<<s2<<"\n";
    }
}
}
return 0;
}