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

const int N = 52;

int c1[N], c2[N][N], cnt[N*N], ans[N];
bool vis[N];


bool f2 = 0;

void precompute(int n){
    memset(cnt, 0, sizeof cnt);
    memset(c1, 0, sizeof c1);
    memset(c2, 0, sizeof c2);
    memset(vis, 0, sizeof vis);
    for(int i=0; i<n; i++)for(int j=0; j<n; j++)cnt[i*j]++;
    f2 = 0;
}

void dfs(int u, int n){ 
    if(u >= n){
        f2 = 1;
        return ;
    }
    for(int i=0; i<n; i++){ // match char u -> int i 
        if(vis[i])continue; // int used
        vis[i] = 1;
        if(cnt[i] == c1[u]){
            bool f = 1;
            for(int j=0; j<u; j++){
                // char j : int ans[j]
                // char u : int i 
                if(ans[j] && c2[j][u] != cnt[ans[j]*n+i])f = 0;
                if(i && c2[u][j] != cnt[i*n+ans[j]])f = 0;
            }
            if(f){
                // cout << "char: " << u << " <-> int: " << i << "\n"; 
                ans[u] = i;
                dfs(u+1,n);
                if(f2)break;
            }
        }
        vis[i] = 0;
    }
}

void solve(){
    int n;
    cin >> n;

    precompute(n);
    for(int i=0; i<n; i++){
        for(int j=0; j<n; j++){
            string s;
            cin >> s;
            if(s.length() == 1) c1[s[0]-'a']++;
            else c2[s[0]-'a'][s[1]-'a']++;
        }
    }

    dfs(0, n);
    vector<char>a2(n);
    for(int i=0; i<n; i++)a2[ans[i]] = (char)(i+'a');
    for(char c : a2)cout << c;
    cout << "\n";
}


int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    int t;
    cin >> t;
    while(t--)solve();
}