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

const int N=1000050;
char s[N],ans[N];
int id[N],sa[N],pref[N],tmp[N],tmpId[N];

void Solve(int k,int n){
	s[n+1]='a'-1;
	int tot=n+1;
	for(int i=1;i<=tot;i++)id[i]=s[i]-('a'-2),sa[i]=i;
	sort(sa+1,sa+1+tot,[&](int i,int j){return id[i]<id[j];});
	int c=27;
	for(int j=1;;j<<=1){
		for(int i=0;i<=c;i++)pref[i]=0;
		for(int i=1;i<=tot;i++){
			pref[id[i]]++;
		}
		for(int i=1;i<=c;i++)pref[i]+=pref[i-1];
		for(int i=tot;i>=1;i--){
			if(sa[i]>j){
				//printf("take %i\n",sa[i]-j);
				tmp[pref[id[sa[i]-j]]--]=sa[i]-j;
			}
		}
		for(int i=0;i<j;i++){
			tmp[pref[id[tot-i]]--]=tot-i;
		}
		c=0;
		for(int i=1;i<=tot;i++){
			if(i==1 || id[tmp[i]]!=id[tmp[i-1]] || id[tmp[i]+j]!=id[tmp[i-1]+j]){
				c++;
			}
			tmpId[tmp[i]]=c;
			sa[i]=tmp[i];
			//printf("%i ",sa[i]);
		}
		for(int i=1;i<=tot;i++)id[i]=tmpId[i];
		//printf("\n");
		//printf("id: ");for(int i=1;i<=tot;i++)printf("%i ",id[i]);printf("\n");
		if(c==tot)break;
	}

	//for(int i=1;i<=n;i++)printf("%i ",id[i]);
	//printf(":D\n");
	//for(int i=1;i<=tot;i++)printf("%i ",sa[i]);
	//printf(":D\n");

	int from=1,sz=0;
	for(char c='z';c>='a';c--){
		if(k==0)break;
		while(from<=n && s[from]==c){
			from++;
			ans[++sz]=c;
		}
		vector<pair<int,int>> cand;
		for(int i=from;i<=n;i++){
			int cnt=0;
			if(s[i]==c){
				cnt=1;
				while(i+1<=n && s[i+1]==c){
					i++;
					cnt++;
				}
				cand.pb({cnt,i+1});
			}
		}
		sort(cand.rbegin(),cand.rend());
		for(int i=0,j;i<cand.size();i=j){
			for(j=i;j<cand.size() && cand[i].first==cand[j].first;j++);
			int cnt=j-i;
			if(cnt<k){
				k-=cnt;
				for(int z=0;z<cnt*cand[i].first;z++)ans[++sz]=c;
				from=max(from,cand[i].second);
			}else{
				for(int z=0;z<k*cand[i].first;z++)ans[++sz]=c;
				for(int z=j-1;z>=i;z--){
					if(k>0){
						from=max(from,cand[z].second);
						k--;
					}
				}
				vector<int> suff;
				suff.pb(from);
				for(int z=i;z<j;z++){
					if(cand[z].second>from){
						suff.pb(cand[z].second);
					}
				}
				int best=-1,now=-1;
				for(int z:suff){
					if(now<id[z]){
						best=z;
						now=id[z];
					}
				}
				for(int z=best;z<=n;z++){
					ans[++sz]=s[z];
				}
				k=0;
				break;
			}
		}
	}

	ans[sz+1]=0;
	printf("%s\n",ans+1);

	for(int i=1;i<=tot;i++)id[i]=0,sa[i]=0;
}
int main(){
	int t;
	scanf("%i",&t);
	while(t--){
		int k;
		scanf("%i %s",&k,s+1);
		Solve(k,strlen(s+1));
	}
	return 0;
}