#include <bits/stdc++.h>
using namespace std;

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

void exe() {
    int N, K;
    cin >> N >> K;

    set<pii> e;
    vector<int> deg(N, 0);
    for (int i=0; i<K; i++) {
        int x, y;
        cin >> x >> y;

        e.emplace(x, y);
        e.emplace(y, x);
        deg[x]++, deg[y]++;
    }

    set<int> s;

    auto left = [&](int x) {
        auto it = s.lower_bound(x);

        if (it == s.begin()) return *s.rbegin();
        return *prev(it);
    };

    auto right = [&](int x) {
        auto it = s.upper_bound(x);

        if (it == s.end()) return *s.begin();
        return *it;
    };

    for (int i=0; i<N; i++) s.insert(i);

    vector<int> v, q, le(N, 0), ri(N, 0);

    for (int i=0; i<N; i++) if (!deg[i]) q.push_back(i);

    while (s.size() > 3) {
        int x = q.back();
        q.pop_back();

        v.push_back(x);
        s.erase(x);

        int l = left(x), r = right(x);

        le[x] = l, ri[x] = r;

        if (e.count(pii(l, r))) {
            deg[l]--;
            if (!deg[l]) q.push_back(l);
            deg[r]--;
            if (!deg[r]) q.push_back(r);
        }
    }

    reverse(v.begin(), v.end());

    vector<int> ans(N, -1);

    for (int x: s) ans[x] = 0;
    ans[*s.begin()] = 1;

    for (int x : v) {
        assert(ans[le[x]] != -1);
        assert(ans[ri[x]] != -1);
        if (ans[le[x]] == ans[ri[x]]) ans[x] = !ans[le[x]];
        else ans[x] = 1;
    }

    for (int i=0; i<N; i++) cout << (ans[i] ? 'R': 'B');
    cout << '\n';
}

int main()
{
    ios_base::sync_with_stdio(false); cin.tie(NULL);

    int TC;
    cin >> TC;
    while(TC--)
    {
        exe();
    }
}