Problem p03291 (C++)

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

using namespace std;
using ll = long long;
#define int ll
using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define PB push_back

const ll INF = (1LL << 60);
const int MOD = 1000000007;

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <typename T> bool IN(T a, T b, T x) { return a <= x && x < b; }
template <typename T> T ceil(T a, T b) { return a / b + !!(a % b); }
template <class S, class T>
ostream &operator<<(ostream &out, const pair<S, T> &a) {
  out << '(' << a.first << ',' << a.second << ')';
  return out;
}
template <class T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << '[';
  REP(i, a.size()) {
    out << a[i];
    if (i != a.size() - 1)
      out << ',';
  }
  out << ']';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

signed main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  string s;
  cin >> s;
  int n = s.size();

  VI v1(n), v2(n);
  v1[0] = s[0] == 'A' ? 1 : 0;
  v2[0] = s[0] == '?' ? 1 : 0;
  FOR(i, 1, n) {
    v1[i] += v1[i - 1] + (s[i] == 'A' ? 1 : 0);
    v2[i] += v2[i - 1] + (s[i] == '?' ? 1 : 0);
  }

  VI v3(n), v4(n);
  v3[n - 1] = (s[n - 1] == 'C' ? 1 : 0);
  v4[n - 1] = (s[n - 1] == '?' ? 1 : 0);
  for (int i = n - 2; i >= 0; --i) {
    v3[i] += v3[i + 1] + (s[i] == 'C' ? 1 : 0);
    v4[i] += v4[i + 1] + (s[i] == '?' ? 1 : 0);
  }

  int q = v2[n - 1];
  VI x(3, 1);
  REP(i, q - 3)(x[0] *= 3) %= MOD;
  REP(i, q - 2)(x[1] *= 3) %= MOD;
  REP(i, q - 1)(x[2] *= 3) %= MOD;
  REP(i, q)(x[3] *= 3) %= MOD;

  int ret = 0;
  FOR(i, 1, n - 1) {
    if (s[i] == 'B') {
      (ret += v1[i - 1] * v3[i + 1] % MOD * x[3] % MOD) %= MOD;
      (ret += v1[i - 1] * v4[i + 1] % MOD * x[2] % MOD) %= MOD;
      (ret += v2[i - 1] * v3[i + 1] % MOD * x[2] % MOD) %= MOD;
      (ret += v2[i - 1] * v4[i + 1] % MOD * x[1] % MOD) %= MOD;
    }
    if (s[i] == '?') {
      (ret += v1[i - 1] * v3[i + 1] % MOD * x[2] % MOD) %= MOD;
      (ret += v1[i - 1] * v4[i + 1] % MOD * x[1] % MOD) %= MOD;
      (ret += v2[i - 1] * v3[i + 1] % MOD * x[1] % MOD) %= MOD;
      (ret += v2[i - 1] * v4[i + 1] % MOD * x[0] % MOD) %= MOD;
    }
  }
  cout << ret << endl;

  return 0;
}


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

using namespace std;
using ll = long long;
#define int ll
using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define PB push_back

const ll INF = (1LL << 60);
const int MOD = 1000000007;

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <typename T> bool IN(T a, T b, T x) { return a <= x && x < b; }
template <typename T> T ceil(T a, T b) { return a / b + !!(a % b); }
template <class S, class T>
ostream &operator<<(ostream &out, const pair<S, T> &a) {
  out << '(' << a.first << ',' << a.second << ')';
  return out;
}
template <class T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << '[';
  REP(i, a.size()) {
    out << a[i];
    if (i != a.size() - 1)
      out << ',';
  }
  out << ']';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

signed main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  string s;
  cin >> s;
  int n = s.size();

  VI v1(n), v2(n);
  v1[0] = s[0] == 'A' ? 1 : 0;
  v2[0] = s[0] == '?' ? 1 : 0;
  FOR(i, 1, n) {
    v1[i] += v1[i - 1] + (s[i] == 'A' ? 1 : 0);
    v2[i] += v2[i - 1] + (s[i] == '?' ? 1 : 0);
  }

  VI v3(n), v4(n);
  v3[n - 1] = (s[n - 1] == 'C' ? 1 : 0);
  v4[n - 1] = (s[n - 1] == '?' ? 1 : 0);
  for (int i = n - 2; i >= 0; --i) {
    v3[i] += v3[i + 1] + (s[i] == 'C' ? 1 : 0);
    v4[i] += v4[i + 1] + (s[i] == '?' ? 1 : 0);
  }

  int q = v2[n - 1];
  VI x(4, 1);
  REP(i, q - 3)(x[0] *= 3) %= MOD;
  REP(i, q - 2)(x[1] *= 3) %= MOD;
  REP(i, q - 1)(x[2] *= 3) %= MOD;
  REP(i, q)(x[3] *= 3) %= MOD;

  int ret = 0;
  FOR(i, 1, n - 1) {
    if (s[i] == 'B') {
      (ret += v1[i - 1] * v3[i + 1] % MOD * x[3] % MOD) %= MOD;
      (ret += v1[i - 1] * v4[i + 1] % MOD * x[2] % MOD) %= MOD;
      (ret += v2[i - 1] * v3[i + 1] % MOD * x[2] % MOD) %= MOD;
      (ret += v2[i - 1] * v4[i + 1] % MOD * x[1] % MOD) %= MOD;
    }
    if (s[i] == '?') {
      (ret += v1[i - 1] * v3[i + 1] % MOD * x[2] % MOD) %= MOD;
      (ret += v1[i - 1] * v4[i + 1] % MOD * x[1] % MOD) %= MOD;
      (ret += v2[i - 1] * v3[i + 1] % MOD * x[1] % MOD) %= MOD;
      (ret += v2[i - 1] * v4[i + 1] % MOD * x[0] % MOD) %= MOD;
    }
  }
  cout << ret << endl;

  return 0;
}