#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
using namespace std;
const int N=1000005;
struct node
{int l,r;
}a[N];
vector<node> vct[30];
int t,K,n,m,i,j,k,l,x,y,sa[N],rk[N],Y[N],c[N],len;
char s[N],ans[N];
bool cmp(node a,node b)
{return a.l<b.l;
}
bool cmq(node a,node b)
{if(a.r-a.l!=b.r-b.l)return a.r-a.l>b.r-b.l;return a.l<b.l;
}
void add(char c,int x)
{while(x--)ans[++len]=c;
}
int main()
{freopen("string.in","r",stdin);freopen("string.out","w",stdout);scanf("%d",&t);while(t--){scanf("%d%s",&K,s+1);if(K==0)//注意K==0时要特判{puts(s+1);continue;}n=strlen(s+1);m=300;len=0;for(i=1;i<=n;++i)rk[i]=s[i];for(i=1;i<=m;++i)c[i]=0;for(i=1;i<=n;++i)c[rk[i]]++;for(i=1;i<=m;++i)c[i]+=c[i-1];for(i=1;i<=n;++i)sa[c[rk[i]]--]=i;for(k=1;k<=n;k<<=1){for(i=n-k+1,l=0;i<=n;++i)Y[++l]=i;for(i=1;i<=n;++i)if(sa[i]>k)Y[++l]=sa[i]-k;for(i=1;i<=m;++i)c[i]=0;for(i=1;i<=n;++i)c[rk[i]]++;for(i=2;i<=m;++i)c[i]+=c[i-1];for(i=l;i>=1;--i)sa[c[rk[Y[i]]]--]=Y[i];for(i=1;i<=n;++i)Y[i]=rk[i];rk[sa[1]]=m=1;for(i=2;i<=n;++i)rk[sa[i]]=(Y[sa[i]]==Y[sa[i-1]]&&(sa[i]+k>n?0:Y[sa[i]+k])==(sa[i-1]+k>n?0:Y[sa[i-1]+k])?m:++m);if(rk[sa[n]]==n)//SA的一个优化break;}rk[n+1]=0;//注意细节！！for(i=0;i<26;++i)vct[i].clear();for(i=1;i<=n;i=j){for(j=i;j<=n&&s[i]==s[j];++j);vct[s[i]-'a'].push_back((node){i,j-1});}for(i=25,j=k=0;i>=0;--i){l=vct[i].size();if(!l)continue;sort(vct[i].begin(),vct[i].end(),cmp);for(x=0;x<l&&vct[i][x].l<=j;++x);if(x>=l)continue;if(vct[i][x].l==j+1){add(i+'a',vct[i][x].r-vct[i][x].l+1);j=vct[i][x++].r;}if(l-x+k<K){j=vct[i][l-1].r;k+=l-x;for(;x<l;++x)add(i+'a',vct[i][x].r-vct[i][x].l+1);continue;}sort(vct[i].begin()+x,vct[i].end(),cmq);for(;k<K-1;++x){add(i+'a',vct[i][x].r-vct[i][x].l+1);++k;j=max(j,vct[i][x].r);}k=vct[i][x].r-vct[i][x].l+1;add(i+'a',k);for(y=n+1;x<l&&vct[i][x].r-vct[i][x].l+1==k;++x)if(y==n+1||rk[max(j+1,vct[i][x].r+1)]>rk[y])y=max(j+1,vct[i][x].r+1);for(;y<=n;++y)add(s[y],1);j=n;break;}ans[len+1]=0;puts(ans+1);}return 0;
}