A. Do Not Be Distracted!

#include <bits/stdc++.h>

using namespace std ;
#define sz(s) int(s.size())
typedef long long ll ;


void ABDEL3AL () {
    ios_base::sync_with_stdio(false); cin.tie(nullptr) , cout.tie(nullptr) ;
    #ifndef ONLINE_JUDGE
    freopen("input.txt" , "r" , stdin) , freopen("output.txt" , "w" , stdout) ;
    #endif

}

int main() {
    ABDEL3AL() ;
    int t ; cin >> t ;
    while (t--) {
        int n;
        cin >> n;
        string s;
        cin >> s;
        bool vis[26], var = true;
        for (int i = 0; i < sz(s); i++) {
            if (vis[s[i] - 'A']) var = false ;
            if (s[i] != s[i + 1]) vis[s[i] - 'A'] = true ;
        }
        cout << (var ? "YES" : "NO") << "\n" ;
    }

    return 0;
}