#include <bits/stdc++.h>

using namespace std;

char digit_to_char(int x) {
	if(x>=26) return 'A'+x-26;
	return 'a'+x;
}
void print_square(int n) {
	for(int i=0; i<n; ++i) {
		for(int j=0; j<n; ++j) {
			int t = i*j;
			if(t<n) cout<<' ';
			else cout<<digit_to_char(t/n);
			cout<<digit_to_char(t%n)<<' ';
		}
		cout<<'\n';
	}
}

vector<string> gen_ordered(int n) {
	vector<string> in;
	for(int i=0; i<n; ++i) {
		for(int j=0; j<n; ++j) {
			int t = i*j;
			string oo;
			if(t>=n) oo.push_back(digit_to_char(t/n));
			oo.push_back(digit_to_char(t%n));
			in.push_back(oo);
		}
	}
	return in;
}

vector<array<int,4> > stats(vector<string> in) {
	vector<array<int,4> > v(256,{0,0,0,0});
	vector<int> h20(256,0);
	vector<int> h21(256,0);
	vector<int> h1(256,0);
	vector<int> ha(256,0);
	for(auto &s: in) {
		if(s.size()==2) {
			v[s[0]][0]++;
			v[s[1]][1]++;
		} else v[s[0]][2]++;
	}
	for(auto &s: in) {
		v[s[0]][3] += v[s[1]][0];
	}
	return v;
}

string solve(int n, vector<string> in) {
	vector<array<int,4> > si = stats(in);
	vector<array<int,4> > sr = stats(gen_ordered(n));

	string res(n,'+');
	for(int i=0; i<n; ++i) {
		for(int j=0; j<n; ++j) {
			char a=digit_to_char(i);
			char b=digit_to_char(j);
			if(si[a]==sr[b]) res[j]=a;
		}
	}
	return res;
}

void test(int n) {
	vector<string> in;
	for(int i=0; i<n; ++i) {
		for(int j=0; j<n; ++j) {
			int t = i*j;
			string oo;
			if(t>=n) oo.push_back(digit_to_char(t/n));
			oo.push_back(digit_to_char(t%n));
			in.push_back(oo);
		}
	}
	string ans = solve(n,in);
	bool correct=1;
	for(int i=0; i<n; ++i) if(ans[i]!=digit_to_char(i)) correct=0;
	if(!correct) cerr<<"incorrect "<<n<<' '<<ans<<endl;
}

int main() {
	cin.tie(0);
	ios_base::sync_with_stdio(0);
	int t;
	cin>>t;
	while(t--) {
		int n;
		cin>>n;
		vector<string> v(n*n);
		for(int i=0; i<n*n; ++i) cin>>v[i];
		cout<<solve(n,v)<<'\n';
	}
	return 0;
}