#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<map>
#define Redge(u) for (int k = h[u],to; k; k = ed[k].nxt)
#define REP(i,n) for (int i = 1; i <= (n); i++)
#define mp(a,b) make_pair<int,int>(a,b)
#define cls(s) memset(s,0,sizeof(s))
#define cp pair<int,int>
#define LL long long int
using namespace std;
const int maxn = 455,maxm = 1 << 22,INF = 1000000000;
inline int read(){
    int out = 0,flag = 1; char c = getchar();
    while (c < 48 || c > 57){if (c == ‘-‘) flag = -1; c = getchar();}
    while (c >= 48 && c <= 57){out = (out << 3) + (out << 1) + c - 48; c = getchar();}
    return out * flag;
}
int nxt[maxn][26],last[26],f[maxm],n,m;
char S[maxn];
int main(){
    int T = read();
    while (T--){
        n = read(); scanf("%s",S + 1); m = strlen(S + 1);
        if (n > 21){puts("NO"); continue;}
        for (int i = 0; i < n; i++) nxt[m + 1][i] = last[i] = m + 1;
        for (int i = m; ~i; i--){
            for (int j = 0; j < n; j++)
                nxt[i][j] = last[j];
            if (i) last[S[i] - ‘a‘] = i;
        }
        int maxv = (1 << n) - 1;
        for (int i = 1; i <= maxv; i++){
            f[i] = 0;
            for (int j = 0; j < n; j++)
                if (i & (1 << j)){
                    f[i] = max(f[i],nxt[f[i ^ (1 << j)]][j]);
                }
        }
        puts(f[maxv] <= m ? "YES" : "NO");
    }
    return 0;
}