#include<bits/stdc++.h>
using namespace std;
#define ll long long int 
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define mod 1000000007
#define inf 1000000000


ll gcd(ll a, ll b){
    ll temp = max(a,b);
    b = min(a,b); a = temp;

    while(b>0){
        temp = a%b;
        a = b;
        b = temp;
    }
    return a;
}

ll mod_exp(ll x, ll y){
    ll res = 1;
    x = x%mod;

    while(y>0){
        if(y&1) res = (res*x)%mod;
        x = (x*x)%mod;
        y = y/2;
    }

    return res;
}

ll is_palindrome(string s, ll n){
    for(ll i=0; i<n/2;i++){
        if(s[i]!=s[n-i-1]) return 0;
    }

    return 1;
}

bool compare(vector<ll> a, vector<ll> b){
    if(a[1]<b[1]) return true;
    else if(a[1]==b[1]){
        if(a[0]<=b[0]) return true;
        return false;
    }
    return false;
}

ll binSearch(vector<ll> a, ll k){
    ll l=0, r = a.size()-1, mid;

    while(l<=r){
        mid = (l+r)/2;
        if(a[mid]==k){
            while(mid>=0 && a[mid]==k ) mid--;
            return mid;
        }
        else if(a[mid]>k) r = mid-1;
        else l = mid + 1;
    }
    return r;
}

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);

    #ifndef ONLINE_JUDGE
    freopen("D:\\CP_codes\\input.txt", "r", stdin);
    freopen("D:\\CP_codes\\output.txt", "w", stdout);
    #endif // ONLINE_JUDGE

    ll t,n,i,j,count=0,flag=0;
    cin>>t;
    while(t--){
        flag = 0;
        cin>>n;
        string s;
        cin>>s;

        ll cnt[26] = {0};

        for(i=0;i<n;i++) cnt[s[i]-'a']++;

        for(i=0;i<26;i++){
            if(cnt[i]==0){ cout<<char(i+97)<<"\n"; flag=1; break;}
        }

        if(flag) continue; 

        ll arr[700] = {0};

        for(i=0;i<n-1;i++){
            arr[ (s[i]-'a')*26 + (s[i+1]-'a') ]++;
        }

        for(i=0;i<676;i++){
            if(arr[i]==0){
                cout<<char(i/26 + 97)<<char(i%26 + 97)<<"\n"; 
                flag = 1; break;
            }
        }

        if(flag) continue; 

        ll arr2[20000] = {0};

        for(i=0;i<n-2;i++){
            arr2[ (s[i]-'a')*26*26 + (s[i+1]-'a')*26 + (s[i+2]-'a') ]++;
        }

        for(i=0;i<17576;i++){
            if(arr2[i]==0){
                cout<<char(i/(26*26) + 97)<<char((i/26)%26 + 97)<<char(i%26 + 97)<<"\n";
                flag = 1; break;
            }
        }

    }


}