#include <bits/stdc++.h>

#define f first
#define s second
#define pb push_back
#define mp make_pair

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int N = 2000500, inf = 1e9, mod = 998244353;
const ll INF = 1e18;

int sum(int a, int b) {
    a += b;
    if (a >= mod)
        a -= mod;
    return a;
}

int sub(int a, int b) {
    a -= b;
    if (a < 0)
        a += mod;
    return a;
}

int mult(int a, int b) {
    return 1ll * a * b % mod;
}

int bp(int a, int b) {
    int res = 1;
    while (b) {
        if (b & 1)
            res = mult(res, a);
        a = mult(a, a);
        b >>= 1;
    }
    return res;
}

int inv(int x) {
    return bp(x, mod - 2);
}

vector<int> g[N], gr[N];
vector<int> gd[N];

int n, m;

bool was[N];
int depth[N];

char ans[N];

map<int, pii> mt[N];
vector<int> pre[N], nxt[N];
int ls[N], rs[N];

void dfs(int v) {
    if (was[v])
        return;
    was[v] = 1;
    for (auto to: gr[v]) {
        dfs(to);
        depth[v] = depth[to] + 1;
    }
    if (gr[v].empty())
        depth[v] = 1;
}

void norm(vector<int> &v) {
    while (v.size() > 5)
        v.pop_back();
    for (int i = 0; i + 1 < v.size(); i++)
        if (v[i] == v.back()) {
            v.pop_back();
            return;
        }
}

void set_link(int v, int u) {
    if (v == 0 || u == 0)
        return;
    nxt[v].pb(u);
    pre[u].pb(v);
    norm(nxt[v]);
    norm(pre[u]);
}

void f1(int v) {
    if (pre[v].empty() && nxt[v].empty()) {
        int u1 = gr[v][0];
        int u2 = gr[v][gr[v].size() - 1];
        ls[v] = u1;
        rs[v] = u2;
        set_link(u1, u2);
        return;
    }
    for (auto pv: pre[v]) {
        pii x = mt[v][pv];
        int u = x.f;
        if (x.s != 0) {
            if (rs[v] == 0)
                continue;
            u = rs[v] ^ x.f ^ x.s;
        }
        assert(u != 0);
        set_link(ls[pv], u);
        set_link(u, rs[v]);
        ls[v] = u;
        rs[pv] = u;
    }
    for (auto nv: nxt[v]) {
        pii x = mt[v][nv];
        int u = x.f;
        if (x.s != 0) {
            if (ls[v] == 0)
                continue;
            u = ls[v] ^ x.f ^ x.s;
        }
        assert(u != 0);
        set_link(ls[v], u);
        set_link(u, rs[nv]);
        rs[v] = u;
        ls[nv] = u;
    }

}

void f2(int v) {
    if (depth[v] == 1)
        return;
    if (ls[v] != 0 && rs[v] != 0)
        return;
    if (ls[v] == 0 && rs[v] == 0)
        return;
    if (gr[v].size() == 1) {
        int u = gr[v][0];
        set_link(u, u);
        ls[v] = rs[v] = u;
    } else if (gr[v].size() == 2) {
        int u1 = ls[v] | rs[v];
        int u2 = gr[v][0] ^ gr[v][1] ^ u1;
        if (ls[v] == 0)
            ls[v] = u2;
        else
            rs[v] = u2;
    } else
        assert(0);
    set_link(ls[v], rs[v]);
}

void f3(int v) {
    if (ls[v] == 0 && rs[v] == 0) {
        int u1 = gr[v][0];
        int u2 = gr[v][gr[v].size() - 1];
        ls[v] = u1;
        rs[v] = u2;
    }
    set_link(ls[v], rs[v]);
}

int ftimer = 0;
int fwas[N], fdp[N];

int k;

void flex(int v, bool flag) {
    if (depth[v] == 1) {
        if(fwas[v] < ftimer){
            fwas[v] = ftimer;
            ans[v] = 'a' + k;
            k++;
        }
        cout << ans[v];
        return;
    }
    if (flag) {
        flex(ls[v], 1);
        flex(rs[v], 0);
        ans[v] = ans[rs[v]];
    } else {
        if (fwas[v] == ftimer) {
            cout << ans[v];
        } else {
            fwas[v] = ftimer;
            flex(rs[v], 0);
            ans[v] = ans[rs[v]];
        }
    }
}

void solve() {
    ftimer++;
    k = 0;
//    string s = "";
//    for(int i = 0; i < 10; i++)
//    n = 0;
//    map<string, int> a;
//    set<pii> b;
//    for (int i = 0; i < s.size(); i++) {
//        string t = "";
//        for (int j = i; j < s.size(); j++) {
//            t += s[j];
//            if (a.find(t) == a.end()) {
//                n++;
//                a[t] = n;
//            }
//        }
//    }
//    for (int i = 0; i < s.size(); i++) {
//        string t = "";
//        for (int j = i; j < s.size(); j++) {
//            t += s[j];
//            for (char q = 'a'; q <= 'z'; q++) {
//                if (a.find(t + q) != a.end())
//                    b.insert({a[t], a[t + q]});
//                if (a.find(q + t) != a.end())
//                    b.insert({a[t], a[q + t]});
//            }
//        }
//    }
//    n = a.size();
//    m = b.size();
//    for (int i = 0; i < m; i++) {
//        int v, u;
//        cin >> v >> u;
//        v = b.begin()->f;
//        u = b.begin()->s;
//        b.erase(b.begin());
////        cerr << v << " " << u << endl;
//        g[v].pb(u);
//        gr[u].pb(v);
//    }
    cin >> n >> m;
    for (int i = 0; i < m; i++) {
        int v, u;
        cin >> v >> u;
        g[v].pb(u);
        gr[u].pb(v);
    }
    for (int i = 1; i <= n; i++)
        if (!was[i])
            dfs(i);
    for (int i = 1; i <= n; i++)
        gd[depth[i]].pb(i);

    for (int i = 1; i <= n; i++)
        assert(g[i].size() <= 52);

    for (int i = 1; i <= n; i++)
        for (auto x: g[i])
            for (auto y: g[i]) {
                if (mt[x][y].f != 0)
                    mt[x][y].s = i;
                else
                    mt[x][y].f = i;
            }

    int root = -1;

    for (int d = n; d >= 1; d--)
        if (!gd[d].empty()) {
            root = gd[d][0];
            break;
        }

    for (int d = n; d >= 2; d--) {
        while (!gd[d].empty()) {
            bool found = 0;
            for (int i = 0; i < gd[d].size(); i++) {
                int v = gd[d][i];
                f1(v);
                f2(v);
            }
            for (int i = 0; i < gd[d].size(); i++) {
                int v = gd[d][i];
                f1(v);
                f2(v);
                if (ls[v] > 0 && rs[v] > 0) {
                    found = 1;
                    swap(gd[d][i], gd[d][gd[d].size() - 1]);
                    gd[d].pop_back();
                    i--;
                }
            }
            if (!found) {
                f3(gd[d][0]);
            }
        }
    }
//    for (int i = 1; i <= n; i++) {
//        cerr << endl;
//        cerr << "V " << i << endl;
//        cerr << "ls rs " << ls[i] << " " << rs[i] << endl;
//        cerr << "PRE: ";
//        for (auto x: pre[i])
//            cerr << x << " ";
//        cerr << endl;
//        cerr << "NXT: ";
//        for (auto x: nxt[i])
//            cerr << x << " ";
//        cerr << endl;
//    }
    for (int i = 0; i < gd[1].size(); i++)
        ans[gd[1][i]] = 'a' + i;
    flex(root, 1);
    cout << "\n";


    for (int i = 1; i <= n; i++) {
        was[i] = 0;
        ls[i] = rs[i] = 0;
        g[i].clear();
        gr[i].clear();
        gd[i].clear();
        pre[i].clear();
        nxt[i].clear();
        mt[i].clear();
    }
}

int main() {
#ifdef DEBUG
    freopen("input.txt", "r", stdin);
#endif
    ios_base::sync_with_stdio(false);
    int t = 1;
    cin >> t;
    for (int i = 1; i <= t; i++) {
//        cout << "Case #" << i << endl;
        solve();
    }
}