#include<bits/stdc++.h>
using namespace std;

#define LL long long int
#define L long int
#define IM INT_MAX
#define Im INT_MIN
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define read  freopen("input.txt", "r", stdin)
#define write freopen("output.txt", "w", stdout)
#define pb(x) push_back(x)
#define all(v) sort(v.begin(),v.end())
#define For(j,n) for(int i=j;i<n;i++)


//template<class T>inline bool rd(T &x){int c=getchar();int sgn=1;while(~c&&c<'0'||c>'9'){if(c=='-')sgn=-1;c=getchar();}for(x=0;~c&&'0'<=c&&c<='9';c=getchar())x=x*10+c-'0'; x*=sgn; return ~c;}


int main(){

int t;

cin>>t;

while(t--){
    string c;
    int n;
    cin>>n;
    cin>>c;
    map<char,int>mp;
    for(int i=0;i<n;i++){
        mp[c[i]]++;
    }

    map<char,int>::iterator itr;

    for(itr=mp.begin();itr!=mp.end();itr++){
        for(int i=0;i<itr->second;i++)cout<<itr->first;
    }
    cout<<endl;



}
return 0;
}
