#include<cstdio>void up(int& a,int b){	a=a<b?b:a;}int test,n,m,i,j;char t[500];int f[1<<21],s[500][21];int main(){	scanf("%d",&test);	while(test--){		scanf("%d%s",&n,t);		if(n>21)puts("NO");		if(n>21)continue;		for(m=0;t[m];++m)			t[m]-=97;		for(j=0;j<n;++j)			s[m+1][j]=s[m][j]=m+1;		for(i=m-1;~i;--i)			for(j=0;j<n;++j)				s[i][j]=j==t[i]				?i+1:s[i+1][j];		for(i=1;i<1<<n;++i)			for(f[i]=j=0;j<n;++j)				if(i&1<<j)up(f[i],				s[f[i^1<<j]][j]);		puts(f[~(~0<<n)]		<=m?"YES":"NO");	}}