using namespace std;
#include <bits/stdc++.h>
// genrate all string of length k
int k;
int n;
string s;
map<string, int> have;
bool ok = 0;
string ans;
void rec(string ss)
{
  //cerr<<ss<<endl;
  if (ss.length() == k)
  {
    if (have[ss] == 0)
    {
      ok = 1;
      if (ans.length() == 0)
        ans = ss;
      else if (ss < ans)
        ss = ans;
    }
    return ;
  }
  for (int i = 0; i < 26; i++)
  {
    string fake = ss;
    fake += char(i + 'a');
    rec(fake);
  }
}
void solve()
{
  cin>>n;
  cin>>s;
  have.clear();
  ans.clear();
  ok=0;
  for (int i = 0; i < n; i++)
  {
    string st;
    for (int j = i; j < n; j++)
    {
      st += s[j];
      have[st] = 1;
     // cout<<st<<endl;
    }
  }
  for(int len=1;len<1000;len++)
  {
    string fake;
    k=len;
    rec(fake);
    if(ok)
    {
      cout<<ans<<endl;
      return ;
    }
  }
}
int main()
{
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  int t = 1;
  cin >> t;
  while (t--)
    solve();
  return 0;
}
