#include <bits/stdc++.h>

using namespace std;

#define pii pair<int, int>
#define x first
#define y second

#define ll long long

const int N = 55;
char ff[505]; int f[N];

int n; bool fl;

pii str[N * N], num[N * N];

int nm1[N * N], nm2[N * N];

int ton[N][4], ton2[N][4]; 

int a[N];

ll pown2[4], has[N], has2[N];
map<ll, vector<int>> mp;
int vis[N], fvis[N]; // 字母->数字

void calc(int x, int id) {
    int fi = x / n, se = x % n;
    num[id] = make_pair(fi, se);
    if (!fi) ++ton[se][0];
    else {
        if (fi == se) ++ton[fi][3];
        else ++ton[fi][1], ++ton[se][2];
    }
}

void calcstr(int id) {
    int fi = str[id].x, se = str[id].y;
    if (fi == -1) ++ton2[se][0];
    else if (fi == se) ++ton2[fi][3];
    else ++ton2[fi][1], ++ton2[se][2];
}

bool cmp(int x, int y) {
    return mp[has[x]].size() < mp[has[y]].size();
}

bool cmp2(pii a, pii b) {
    return vis[a.x] * n + vis[a.y] < vis[b.x] * n + vis[b.y];
}

bool check() {
    for (int i = 1; i <= n * n; ++i)
        nm2[i] = ((str[i].x == -1) ? 0 : (vis[str[i].x] * n)) + vis[str[i].y];
    sort(nm2 + 1, nm2 + n * n + 1);
    // for (int i = 1; i <= n * n; ++i) printf("%d ", nm2[i]); putchar('\n');
    for (int i = 1; i <= n * n; ++i)
        if (nm1[i] != nm2[i]) {
            // printf("nm1[%d] = %d, nm2[%d] = %d; \n", i, nm1[i], i, nm2[i]);
            return false;
        }
    return true;
}

void print() {
    for (int i = 0; i < n; ++i) fvis[vis[i]] = i;
    for (int i = 0; i < n; ++i) printf("%c", f[fvis[i]]); putchar('\n');
}

void dfs(int step) {
    if (fl) return;
    if (step == n) {
        // puts("sb");
        if (check()) {
            print();
            fl = 1;
        }      
        return;
    }
    for (auto x : mp[has[a[step]]]) {
        if (vis[x]) continue;
        vis[x] = step;
        dfs(step + 1);
        vis[x] = 0;
    }
}

void work() {
    scanf("%d", &n);
    for (int i = 0; i < n; ++i)
        for (int j = 0; j < 4; ++j) ton[i][j] = ton2[i][j] = 0;
    for (int i = 1; i <= n * n; ++i) {
        char s[4];
        scanf("%s", s + 1);
        int len = strlen(s + 1);
        if (len == 1) str[i].x = -1, str[i].y = ff[s[1]];
        else str[i].x = ff[s[1]], str[i].y = ff[s[2]];
        calcstr(i);
    }
    for (int i = 0; i < n; ++i)
        for (int j = 0; j < n; ++j)
            calc(i * j, i * n + j + 1), nm1[i * n + j + 1] = i * j;
    sort(nm1 + 1, nm1 + n * n + 1);
    // for (int i = 1; i <= n * n; ++i) printf("%d ", nm1[i]); putchar('\n');
    pown2[0] = 1; for (int i = 1; i < 4; ++i) pown2[i] = pown2[i - 1] * (n * n + 1);
    for (int i = 0; i < n; ++i) {
        has[i] = 0;
        for (int j = 0; j < 4; ++j) has[i] += pown2[j] * ton[i][j];
        // printf("i = %d, {%d, %d}, hash = %lld; {%d, %d, %d, %d} \n", i, num[i].x, num[i].y, has[i], ton[i][0], ton[i][1], ton[i][2], ton[i][3]);
    }  
    for (int i = 0; i < n; ++i) {
        has2[i] = vis[i] = 0;
        for (int j = 0; j < 4; ++j) has2[i] += pown2[j] * ton2[i][j];
        // printf("i = %d, {%d, %d}, hash = %lld; {%d, %d, %d, %d} \n", i, str[i].x, str[i].y, has2[i], ton2[i][0], ton2[i][1], ton2[i][2], ton2[i][3]);
        mp[has2[i]].push_back(i); 
    }
    for (int i = 0; i < n; ++i) a[i] = i;
    sort(a, a + n, cmp);
    dfs(1); mp.clear(); fl = 0;
}

int main() {
    for (int i = 'a'; i <= 'z'; ++i) ff[i] = i - 'a', f[i - 'a'] = i;
    for (int i = 'A'; i <= 'Z'; ++i) ff[i] = i - 'A' + 26, f[i - 'A' + 26] = i;
    int Tt; scanf("%d", &Tt);
    while (Tt--) work();
}