#include<cstdio>

 #include<cstring>

 #include<algorithm>

 #define rep(i,l,r) for (int i=(l); i<=(r); i++)

 using namespace std;

 int T,n,m,k,t,g[][],f[<<];

 char a[];

 int main(){

     freopen("bzoj4416.in","r",stdin);

     freopen("bzoj4416.out","w",stdout);

     scanf("%d",&T);

     while(T--){

         scanf("%d%s",&n,a+); m=strlen(a+);

         if (n>){ puts("NO"); continue; }

         rep(j,,n-) g[m][j]=g[m+][j]=m+;

         for(int i=m; i; i--){

             rep(j,,n-) g[i-][j]=g[i][j];

             g[i-][a[i]-'a']=i;

         }

         rep(i,,(<<n)-){

             int res=;

             for(int j=i; j; j-=j&-j)

                 k=__builtin_ctz(j),res=max(res,g[f[i^(<<k)]][k]);//ctz统计末尾0的个数

             f[i]=res;

         }

         puts(f[(<<n)-]>m ? "NO" : "YES");

     }

     return ;

 }