#include<iostream>
#include<cstring>
#include<vector>
#include<map>
using namespace std;
using LL = long long;

int main(){

#ifdef LOCAL
    freopen("data.in", "r", stdin);
    freopen("data.out", "w", stdout);
#endif

    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(0);

    auto get = [&](char c){
        if (islower(c)) return c - 'a';
        return 26 + c - 'A';
    };

    int T;
    cin >> T;
    while(T--){
        int n;
        cin >> n;
        vector<vector<pair<int, int> > > pos(n);
        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                int x = i * j;
                vector<int> v;
                while(x){
                    v.push_back(x % n);
                    x /= n;
                }
                if (v.empty()) v.push_back(0);
                reverse(v.begin(), v.end());
                for(int k = 0; k < v.size(); k++){
                    pos[v[k]].push_back({v.size(), k});
                }
            }
        }
        map<vector<pair<int, int> >, vector<int> > mp;
        for(int i = 0; i < n; i++){
            sort(pos[i].begin(), pos[i].end());
            mp[pos[i]].push_back(i);
        }
        vector<vector<pair<int, int> > > cur(n);
        for(int i = 0; i < n * n; i++){
            string s;
            cin >> s;
            for(int j = 0; j < s.size(); j++){
                cur[get(s[j])].push_back({s.size(), j});
            }
        }
        string ans(n, '.');
        for(int i = 0; i < n; i++){
            sort(cur[i].begin(), cur[i].end());
            auto &v = mp[cur[i]];
            ans[v.back()] = i < 26 ? char('a' + i) : char('A' + i - 26);
            v.pop_back();
        }
        cout << ans << '\n';
    }

}