/* BISMILLAH*/
#include<bits/stdc++.h>
using namespace std;
#define FAST ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define ll long long
#define all(v) v.begin(),v.end()
#define YES cout<<"YES\n"
#define NO  cout<<"NO\n"
#define pii pair<int,int>
#define pll pair<ll,ll>
#define vi vector<int>
#define vl vector<ll>
#define vpi vector<pii>
#define vpl vector<pll>
#define first ff
#define second ss
#define pb push_back
void solve()
{
    int n;
    string s;
    cin >> n >> s;
    set<string> se;
    for(int i=0;i<n;i++) {
        string ss;
        for(int k = 0; k < 5 && i + k < n; k++) {
            ss.push_back(s[i + k]);
            se.insert(ss);
        }
    }
    for(int len=1;len<6;len++) {
        string t(len, 'a');
        while(true) {
            if(se.count(t) == 0) {
                cout << t << '\n';
                return;
            }
            int idx = len - 1;
            while(idx >= 0 && t[idx] == 'z') {
                t[idx] = 'a';
                idx--;
            }
            if(idx < 0) break;
            t[idx]++;
        }
    }
}
int main()
{
   #ifndef ONLINE_JUDGE
     freopen("input.txt","r",stdin);
     freopen("output.txt","w",stdout);
   #endif
   FAST
   int t;
   cin>>t;
   while(t--)
       solve();
}
