#include <bits/stdc++.h>
#define debug cerr << "\033[33m[" << __LINE__ << "]\033[0m "
#define SZ(x) (int) x.size() - 1
#define all(x) x.begin(), x.end()
#define ms(x, y) memset(x, y, sizeof x)
#define F(i, x, y) for (int i = (x); i <= (y); i++)
#define DF(i, x, y) for (int i = (x); i >= (y); i--)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
template <typename T> T& chkmax(T &x, T y) {return x = max(x, y);}
template <typename T> T& chkmin(T &x, T y) {return x = min(x, y);}
template <typename T> T& read(T &x) {
	x = 0; int f = 1; char c = getchar();
	for (; !isdigit(c); c = getchar()) if (c == '-') f = -f;
	for (; isdigit(c); c = getchar()) x = (x << 1) + (x << 3) + (c ^ 48);
	return x *= f;
}
const int N = 2e6 + 10;
int n, m, deg[N], len[N], fa[N], g[N], fv[N];
int val[N];
vector <int> v[N], vv[N], s[N], r[N], l[N];
bool vis[N];
int find(int x) {return x == fa[x] ? x : fa[x] = find(fa[x]);}
void merge(int x, int y) {
	x = find(x), y = find(y);
	if (x == y) return;
	fa[x] = y;
	for (int i: r[x]) r[y].push_back(i);
	for (int i: l[x]) l[y].push_back(i);
}
void zhk() {
	read(n), read(m);
	F(i, 1, n) {
		vis[i] = false;
		deg[i] = val[i] = g[i] = 0;
		s[i].clear(), l[i].clear(), r[i].clear(), v[i].clear(), vv[i].clear();
	}
	F(i, 1, m) {
		int x, y; read(x), read(y);
		deg[y]++;
		v[x].push_back(y);
		vv[y].push_back(x);
	}
	F(i, 1, n) sort(all(vv[i]));
	queue <int> q;
	F(i, 1, n)
		if (!deg[i]) q.push(i), len[i] = 1;
	if (q.size() == 1) {
		F(i, 1, n) cout << 'a';
		cout << '\n';
		return;
	}
	int mx = 0;
	while (q.size()) {
		int x = q.front(); q.pop();
		for (int i: v[x]) {
			chkmax(mx, len[i] = len[x] + 1);
			if (--deg[i] == 0) q.push(i);
		}
	}
	F(i, 1, n) s[len[i]].push_back(i);
	fa[1] = 1, fa[2] = 2;
	r[1].push_back(2);
	l[2].push_back(1);
	g[vv[s[mx][0]][0]] = 1;
	g[vv[s[mx][0]][1]] = 2;
	val[1] = vv[s[mx][0]][0];
	val[2] = vv[s[mx][0]][1];
	DF(i, mx - 1, 2) {
		vector <pair <int, int>> e;
		int p = mx - i + 2;
		vis[p] = false;
		fa[p] = p;
		l[p].push_back(find(p - 1));
		r[find(p - 1)].push_back(p);
		for (int j: s[i]) vis[g[j]] = false;
		queue <int> q;
		auto work = [&] (int x, int y) {
			if (vis[y]) return;
			val[y] = x;
			q.push(y);
			vis[y] = true;
			if (g[x]) e.emplace_back(y, g[x]);
			else g[x] = y;
		};
		auto update = [&] (int t, vector <int>& x) {
			vector <int> w;
			for (int k: x) if (find(k) != t) w.push_back(find(k));
			sort(all(w));
			w.resize(unique(all(w)) - w.begin());
			swap(x, w);
		};
		for (int j: s[i]) {
			fv[g[j]] = val[g[j]];
			update(g[j], l[g[j]]);
			update(g[j], r[g[j]]);
		}
		auto solve = [&] (int pos) {
			if (vis[pos]) return;
			int j = val[pos];
			if (vv[j].size() == 1) {
				work(vv[j][0], pos);
				for (int k: r[pos]) work(vv[j][0], k);
			} else {
				for (int k: r[pos]) {
					if (vis[k]) {
						work(vv[j][0] ^ vv[j][1] ^ val[k], pos);
						break;
					}
					if (vv[val[k]].size() == 2 && vv[val[k]] != vv[j]) {
						vector <int> a = vv[j], b = vv[val[k]];
						int tt = -1;
						for (int ta: a) if (tt == -1)
							for (int tb: b) {
								if (ta == tb) {
									tt = ta;
									break;
								}
							}
						int t1 = a[0] ^ a[1] ^ tt, t2 = b[0] ^ b[1] ^ tt;
						work(t1, pos);
						work(tt, k);
						for (int t: r[k]) work(t2, t);
					}
				}
			}
		};
		for (int j: s[i]) solve(g[j]);
		while (q.size()) {
			int x = q.front(), j = fv[x]; q.pop();
			if (x != p) {
				if (vv[j].size() == 1) {
					for (int k: r[x]) work(vv[j][0], k);
				} else {
					for (int k: r[x]) {
						work(vv[j][0] ^ vv[j][1] ^ val[x], k);
					}
				}
			}
			for (int j: l[x]) solve(j);
		}
		for (int j: s[i])
			if (!vis[g[j]]) {
				F(i, 1, mx)
					if (i & 1) cout << 'a';
					else cout << 'b';
				cout << '\n';
				return;
			}
		for (auto [a, b]: e) merge(a, b);
	}
	F(i, 1, mx) val[i] = 0;
	string ans1 = "", ans2 = "";
	int tot = 0;
	F(i, 1, mx) {
		if (!val[find(i)]) val[find(i)] = ++tot;
		ans1 += char('a' + val[find(i)] - 1);
	}
	F(i, 1, mx) val[i] = 0;
	tot = 0;
	DF(i, mx, 1) {
		if (!val[find(i)]) val[find(i)] = ++tot;
		ans2 += char('a' + val[find(i)] - 1);
	}
	cout << min(ans1, ans2) << '\n';
}
signed main() {
	int _ = 1;
	cin >> _;
	while (_--) zhk();
	return 0;
}