#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=1e6+5;
const ll mod=998244353;
vector<int>in[N];
int n,m,k;
int out[N];
int pr[N];

ll w[N];
bool vis[N];
void dfs(int id){
	vis[id]=true;
	for(auto c:in[id]){
		if(!vis[c]) dfs(c);
	}
	if(in[id].empty()) w[id]=id;
	else{
		w[id]=0;
		for(int j=0; j<2 ;j++){
			w[id]=(w[id]+w[in[id][j%in[id].size()]])%mod;
		}
	}
}

ll f[N],inf[N];
ll pw(ll x,ll y){
	if(y==0) return 1;
	if(y%2) return x*pw(x,y-1)%mod;
	ll res=pw(x,y/2);
	return res*res%mod;
}
ll C(ll x,ll y){
	return f[x]*inf[y]%mod*inf[x-y]%mod;
}
int find(vector<int>&v,int c){
	for(int i=0; i<v.size() ;i++){
		if(c==v[i]) return i;
	}
	return -1;
}

vector<int>cyclic(vector<int>g,int p){
	if(p==1) return g;
	vector<int>head;
	{
		for(auto c:in[g[0]]){
			bool ok=true;
			int d=c;
			for(int i=0; i<g.size() ;i++){
				int p=find(in[g[i]],c);
				if(p==-1) ok=false;
				if(in[g[i]].size()==2){
					c^=in[g[i]][0];
					c^=in[g[i]][1];
				}
			}
			if(ok && d==c) head.push_back(c);
		}
	}
	int c=head[0];
	vector<int>ng;
	ng.push_back(c);
	for(int i=0; i<g.size() ;i++){
		if(in[g[i]].size()==2){
			c^=in[g[i]][0];
			c^=in[g[i]][1];
		}
		ng.push_back(c);
	}
	ng.pop_back();
	return cyclic(ng,p-1);
}
vector<int>karina(vector<int>g,int p){//not guarenteed that g is distinct
	/*cout << "karina " << p << endl;
	for(auto c:g) cout << c << ' ';
	cout << endl;*/
	if(p==1) return g;
	vector<int>head;
	{
		for(auto c:in[g[0]]){
			bool ok=true;
			int d=c;
			for(int i=0; i<g.size() ;i++){
				int p=find(in[g[i]],c);
				if(p==-1) ok=false;
				if(in[g[i]].size()==2){
					c^=in[g[i]][0];
					c^=in[g[i]][1];
				}
			}
			if(ok) head.push_back(d);
		}
	}
	int c=head[0];
	vector<int>ng;
	ng.push_back(c);
	for(int i=0; i<g.size() ;i++){
		if(in[g[i]].size()==2){
			c^=in[g[i]][0];
			c^=in[g[i]][1];
		}
		ng.push_back(c);
	}
	return karina(ng,p-1);
	
}
int mem[N];

ll jk[N];
vector<int>&winter(vector<int>g,int p){//guarenteed that g is distinct
	if(g.size()>p) return karina(g,p);
	vector<int>head;
	{
		for(auto c:in[g[0]]){
			bool ok=true;
			int d=c;
			for(int i=0; i<g.size() ;i++){
				int p=find(in[g[i]],c);
				if(p==-1) ok=false;
				if(in[g[i]].size()==2){
					c^=in[g[i]][0];
					c^=in[g[i]][1];
				}
			}
			if(ok) head.push_back(d);
		}
	}
	int c=head[0];
	vector<int>ng;
	ng.push_back(c);
	for(int i=0; i<g.size() ;i++){
		if(in[g[i]].size()==2){
			c^=in[g[i]][0];
			c^=in[g[i]][1];
		}
		ng.push_back(c);
	}
	for(auto c:ng) mem[c]=0;
	int fl=-1,fr=-1;
	for(int i=1; i<ng.size() ;i++){
		if(ng[mem[ng[i]]]==ng[i]){
			fl=mem[ng[i]];
			fr=i;
			break;
		}
	}
	if(fl==-1){
		return winter(ng,p-1);
	}
	vector<int>nk,nc;
	for(int i=0; i<fl ;i++){
		nk.push_back(ng[i]);
	}
	for(int i=fl; i<fr ;i++) nc.push_back(ng[i]);
	for(int i=fr; i<ng.size() ;i++) nk.push_back(ng[i]);
	/*cout << "WINTER" << endl;
	cout << "NC: ";
	for(auto c:nc) cout << c << ' ';
	cout << endl;
	cout << "NK: ";
	for(auto c:nk) cout << c << ' ';
	cout << endl;
	*/
	p--;
	if(nc.size()==2 && nk.size()>2){
		vector<int>fn;
		auto v1=winter(nk,p);
		for(int i=0; i<fl+1 ;i++) fn.push_back(v1[i]);
		for(int i=fl; i<v1.size() ;i++) fn.push_back(v1[i]);
		return fn;
	}
	auto v2=cyclic(nc,p);
	int pr=v2.size();
	for(int i=0; i<p ;i++){
		v2.push_back(v2[i%pr]);
	}
	for(int i=0; i<v2.size() ;i++){
		jk[fl+i]=v2[i];
	}
	vector<int>v3;
	for(int i=fl-1; i>=0 ;i--){
		ll cur=w[ng[i]];
		for(int j=1; j<=p-1 ;j++){
			cur=(cur+(mod-C(p-1,j))*jk[i+j])%mod;
		}
	}
	for(int i=fr+1; i<ng.size() ;i++){
		ll cur=w[ng[i]];
		for(int j=0; j<p-1 ;j++){
			cur=(cur+(mod-C(p-1,j))*jk[i+j])%mod;
		}
		jk[i+p-1]=cur;
	}
	for(int i=0; i<ng.size()+p-1 ;i++){
		v3.push_back(jk[i]);
	}
	return v3;
}

int lb[N];
string greedy(vector<int>s){
	for(int i=1; i<=n ;i++) lb[i]=0;
	int ptr=0;
	string res;
	for(auto c:s){
		//cout << "haha " << c << endl;
		if(lb[c]==0){
			lb[c]=++ptr;
		}
		res+='a'+lb[c]-1;
	}
	return res;
}
void solve(){
	cin >> n >> m;
	for(int i=1; i<=n ;i++){
		in[i].clear();
		out[i]=0;
		vis[i]=false;
	}
	f[0]=1;
	for(int i=1; i<=n ;i++) f[i]=f[i-1]*i%mod;
	inf[n]=pw(f[n],mod-2);
	for(int i=n; i>=1 ;i--) inf[i-1]=inf[i]*i%mod;
	for(int i=1; i<=m ;i++){
		int u,v;cin >> u >> v;
		in[v].push_back(u);
		out[u]++;
	}
	if(n==1){
		cout << "a\n";
		return;
	}
	int boss=0;
	for(int i=1; i<=n ;i++){
		if(out[i]==0){
			boss=i;break;
		}
	}
	dfs(boss);
	{//compute length
		int x=boss;
		k=1;
		while(!in[x].empty()){
			k++;
			x=in[x][0];
		}
	}
	if(in[boss].size()==1){
		for(int i=1; i<=n ;i++) cout << "a";
		cout << '\n';
		return;
	}
	auto v1=winter({in[boss][0],in[boss][1]},k-1);
	string s1=greedy(v1);
	auto v2=winter({in[boss][1],in[boss][0]},k-1);
	string s2=greedy(v2);
	cout << min(s1,s2) << '\n';
}
int main(){
	ios::sync_with_stdio(false);cin.tie(0);
	int t;cin >> t;while(t--) solve();
}

answer.code: In function ‘std::vector<int>& winter(std::vector<int>, int)’:
answer.code:113:37: error: cannot bind non-const lvalue reference of type ‘std::vector<int>&’ to an rvalue of type ‘std::vector<int>’
  113 |         if(g.size()>p) return karina(g,p);
      |                               ~~~~~~^~~~~
answer.code:172:24: warning: reference to local variable ‘fn’ returned [-Wreturn-local-addr]
  172 |                 return fn;
      |                        ^~
answer.code:168:28: note: declared here
  168 |                 vector<int>fn;
      |                            ^~
answer.code:199:16: warning: reference to local variable ‘v3’ returned [-Wreturn-local-addr]
  199 |         return v3;
      |                ^~
answer.code:182:20: note: declared here
  182 |         vector<int>v3;
      |                    ^~