#include <bits/stdc++.h>
using namespace std; 
 
using ll = long long;
using ull = unsigned long long;
using ld = double;
 
constexpr ll INF = 4e18;
constexpr ld EPS = 1e-9;
constexpr ll MOD = 998244353;
const ll mod = 998244353;
 
#define endl '\n'
 
#define debug(x) cout << #x << " = " << x << "\n";
#define vdebug(a) cout << #a << " = "; for(auto x: a) cout << x << " "; cout << "\n";
 
void solve(){
	ll n,m,k,i,j;
	cin >> n >>m;
 
	string s;
	cin>>s;
 
	map <char,int> mpp;
	vector<ll> a;
 
	for (char c : s){
		mpp[c]++;
	}
 
	for(auto i : mpp){
		//cout<<i.second;
		a.push_back(i.second);
	}
 
	for(int j = 0 ; j<n ; j++){
		cout<<a[j];
	}
 
	cout<<endl;
 
}
 
 
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
 
	int t;
	cin>>t;
	while(t--){
		solve();
	}
 
	return 0;
}
 

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

MTQKMSAwCmEKMiAwCmFiCjIgMQpiYQozIDEKYWJiCjMgMgphYmMKNiAyCmJhY2FjZAo2IDIKZmFnYnphCjYgMgp6d2FhZmEKNyAyCnRhYWdhYWsKMTQgMwp0dHJyYWFra3R0b29ycgo1IDMKZGViZGIKNSA0CmVjYWRjCjUgMwpkZWJjYQo1IDMKYWJhYWMK

14
1 0
a
2 0
ab
2 1
ba
3 1
abb
3 2
abc
6 2
bacacd
6 2
fagbza
6 2
zwaafa
7 2
taagaak
14 3
ttrraakkttoorr
5 3
debdb
5 4
ecadc
5 3
debca
5 3
abaac