#include<bits/stdc++.h>
using namespace std;
#define endl "\n"
#define fio       ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define fr(i,a,b)   for(int i=a;i<b;i++)
#define frr(i,a,b)  for(int i=a;i<=b;i++)
#define ll  long long int
#define mem(a,b) memset(a,b,sizeof(a))
#define MOD     1000000007
#define mp       make_pair
#define pb       push_back
#define rep(i,a,b)  for(int i=b;i>=a;i--)
#define SIZE      26
#define tst       ll t1;cin>>t1;while(t1--)
#define vec(v)   vector<ll>v
#define ull      unsigned long long int
#define gcd      __gcd

bool sortbysec(const pair<int,int> &a, 
              const pair<int,int> &b) 
{ 
    return (a.second < b.second); 
}

bool PerfectSquare(long double x)
{
    // Find floating point value of
    // square root of x.
    if (x >= 0) {
 
        long long sr = sqrt(x);
         
        // if product of square root
        //is equal, then
        // return T/F
        return (sr * sr == x);
    }
    // else return false if n<0
    return false;
}




  void solve()
{
  ll n;
  cin>>n;
  string s;
  cin>>s;
  vector<pair<char,ll>> v;
  ll ans=0;
  fr(i,0,n)
  {
    v.pb(mp(s[i],i));
  }
  sort(v.begin(),v.end());
 
  fr(i,1,v.size())
  {
    if(v[i].first==v[i-1].first)
    {
    if(v[i].second!=v[i-1].second+1)
    {
      ans=-1;
      break;
    }
  }
  }
  if(ans==-1)
    cout<<"NO"<<endl;
  else
    cout<<"YES"<<endl;
  }

int main()
{
      #ifndef ONLINE_JUDGE
    //for getting input from input.txt
    freopen("input.txt", "r", stdin);
    //for writing output to output.txt
    freopen("output.txt", "w", stdout);
    #endif

    fio;
        
   tst
   {

    solve();
  }
  



    return 0;
}