codeforce.cpp

#include<bits/stdc++.h>
#include<vector>
using namespace std;

 void solve(){


            int n;

            string s;
            cin >> n;
            cin >>s;
            int c[26] = {0};
            c[s[0]-'A']++;
            for(int i=1; i<n; ++i) {
                if(c[s[i]-'A']) {
                    if(s[i]==s[i-1]) {

                    }
                    else{
                        cout << "NO" << endl;
                    }
                }
                c[s[i]-'A']++;
            }
            cout << "YES"<< endl;
        }

int main() {
    int t;
    cin >> t;
    while(t--) {
        solve();
    }
      }