Problem p02799 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for (ll(i) = (0); (i) < (n); ++i)
#define REV(i, n) for (ll(i) = (n)-1; (i) >= 0; --i)
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define SHOW1d(v, n)                                                           \
  {                                                                            \
    REP(WW, n) cerr << v[WW] << ' ';                                           \
    cerr << endl << endl;                                                      \
  }
#define SHOW2d(v, WW, HH)                                                      \
  {                                                                            \
    REP(W_, WW) {                                                              \
      REP(H_, HH) cerr << v[W_][H_] << ' ';                                    \
      cerr << endl;                                                            \
    }                                                                          \
    cerr << endl;                                                              \
  }
#define ALL(v) v.begin(), v.end()
#define Decimal fixed << setprecision(20)
#define INF 1000000000
#define LLINF 1000000000000000000LL
#define MOD 998244353

typedef long long ll;
typedef pair<ll, ll> P;

vector<vector<P>> v(111111);
ll d[111111];
ll c[222222];
ll color[111111];
ll used[111111];

int main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(false);

  ll n, m;
  cin >> n >> m;
  REP(i, n) cin >> d[i];
  REP(i, m) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    v[a].EB(b, i);
    v[b].EB(a, i);
  }

  queue<ll> q;
  REP(i, n) {
    REP(j, v[i].size()) {
      int aite = v[i][j].FI;
      int edge_id = v[i][j].SE;
      if (d[i] == d[aite]) {
        if (color[i] != 0 && color[aite] != 0)
          continue;
        if (color[i] == 0) {
          if (color[aite] != 0)
            color[i] = -color[aite];
          else
            color[i] = 1;
        }
        if (color[aite] == 0)
          color[aite] = -color[i];
        q.push(i);
        q.push(aite);
        c[edge_id] = d[i];
      }
    }
  }

  while (!q.empty()) {
    int id = q.front();
    q.pop();
    // cout << "id " << id << endl;
    if (used[id])
      continue;
    used[id] = true;
    REP(i, v[id].size()) {
      int aite = v[id][id].FI;
      int edge_id = v[id][id].SE;
      // cout << " " << aite << endl;
      if (used[aite])
        continue;
      if (d[aite] <= d[id])
        continue;
      if (color[aite] != 0)
        continue;
      color[aite] = color[id];
      c[edge_id] = d[aite] - d[id];
      q.push(aite);
    }
  }

  REP(i, n) {
    if (!used[i]) {
      cout << -1 << endl;
      return 0;
    }
  }

  REP(i, n) cout << (color[i] == 1 ? 'B' : 'W');
  cout << endl;
  REP(i, m) cout << (c[i] == 0 ? INF : c[i]) << endl;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for (ll(i) = (0); (i) < (n); ++i)
#define REV(i, n) for (ll(i) = (n)-1; (i) >= 0; --i)
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define SHOW1d(v, n)                                                           \
  {                                                                            \
    REP(WW, n) cerr << v[WW] << ' ';                                           \
    cerr << endl << endl;                                                      \
  }
#define SHOW2d(v, WW, HH)                                                      \
  {                                                                            \
    REP(W_, WW) {                                                              \
      REP(H_, HH) cerr << v[W_][H_] << ' ';                                    \
      cerr << endl;                                                            \
    }                                                                          \
    cerr << endl;                                                              \
  }
#define ALL(v) v.begin(), v.end()
#define Decimal fixed << setprecision(20)
#define INF 1000000000
#define LLINF 1000000000000000000LL
#define MOD 998244353

typedef long long ll;
typedef pair<ll, ll> P;

vector<vector<P>> v(111111);
ll d[111111];
ll c[222222];
ll color[111111];
ll used[111111];

int main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(false);

  ll n, m;
  cin >> n >> m;
  REP(i, n) cin >> d[i];
  REP(i, m) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    v[a].EB(b, i);
    v[b].EB(a, i);
  }

  queue<ll> q;
  REP(i, n) {
    REP(j, v[i].size()) {
      int aite = v[i][j].FI;
      int edge_id = v[i][j].SE;
      if (d[i] == d[aite]) {
        if (color[i] != 0 && color[aite] != 0)
          continue;
        if (color[i] == 0) {
          if (color[aite] != 0)
            color[i] = -color[aite];
          else
            color[i] = 1;
        }
        if (color[aite] == 0)
          color[aite] = -color[i];
        q.push(i);
        q.push(aite);
        c[edge_id] = d[i];
      }
    }
  }

  while (!q.empty()) {
    int id = q.front();
    q.pop();
    // cout << "id " << id << endl;
    if (used[id])
      continue;
    used[id] = true;
    REP(i, v[id].size()) {
      int aite = v[id][i].FI;
      int edge_id = v[id][i].SE;
      // cout << " " << aite << endl;
      if (used[aite])
        continue;
      if (d[aite] <= d[id])
        continue;
      if (color[aite] != 0)
        continue;
      color[aite] = color[id];
      c[edge_id] = d[aite] - d[id];
      q.push(aite);
    }
  }

  REP(i, n) {
    if (!used[i]) {
      cout << -1 << endl;
      return 0;
    }
  }

  REP(i, n) cout << (color[i] == 1 ? 'B' : 'W');
  cout << endl;
  REP(i, m) cout << (c[i] == 0 ? INF : c[i]) << endl;

  return 0;
}