#include <bits/stdc++.h>
 
#ifdef LOCAL
#include "brxteforceman/debug.h"
#else
#define debug(...)
#endif
 
void solve(int t) {
  int n;
  std::cin >> n;
  std::string s;
  std::cin >> s;
 
  std::vector<int> cnt(26, 0);
  for (int i = 0; i < n; ++i)
    cnt[s[i] - 'a']++;
 
  std::string ans = "";
  int j = 0;
  for (int i = 0; i < n; ++i) {
    while (cnt[j] == 0) {
      j++;
      j %= 26;
    }
    ans.push_back(j + 'a');
    cnt[j++]--;
    j %= 26;
  }
 
  std::cout << ans << "\n";
}
 
signed main(void) {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  auto precompute = [&]() {};
  precompute();
  int t = 1;
  std::cin >> t;
  for (int i = 1; i <= t; ++i) {
    std::string _t = "testcase (" + std::to_string(i) + ")\n";
    debug(_t);
    solve(i);
  }
}