#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


int N, M;
vector<int> A, B;

int main() {
  for (int numCases; ~scanf("%d", &numCases); ) { for (int caseId = 1; caseId <= numCases; ++caseId) {
    scanf("%d%d", &N, &M);
    A.resize(M);
    B.resize(M);
    for (int i = 0; i < M; ++i) {
      scanf("%d%d", &A[i], &B[i]);
    }
    
    vector<vector<int>> graph(N);
    for (int i = 0; i < M; ++i) {
      graph[A[i]].push_back(B[i]);
      graph[B[i]].push_back(A[i]);
    }
    queue<int> que;
    vector<int> deg(N);
    for (int u = 0; u < N; ++u) {
      deg[u] = graph[u].size();
      if (deg[u] <= 2) {
        que.push(u);
      }
    }
    vector<int> us;
    for (; que.size(); ) {
      const int u = que.front();
      que.pop();
      us.push_back(u);
      for (const int v : graph[u]) {
        if (--deg[v] == 2) {
          que.push(v);
        }
      }
    }
// cerr<<"us = "<<us<<endl;
    assert((int)us.size() == N);
    
    string ans(N, '?');
    for (int j = N; --j >= 0; ) {
      const int u = us[j];
      int cnt[2] = {};
      for (const int v : graph[u]) {
        if (ans[v] == 'B') ++cnt[0];
        if (ans[v] == 'R') ++cnt[1];
      }
      ans[u] = cnt[0] ? 'R' : 'B';
    }
    puts(ans.c_str());
  }
#ifndef LOCAL
  break;
#endif
  }
  return 0;
}