Problem p03626 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define dump(...)
#endif
#define endl "\n"
#define ll long long
#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define REP(i, x) for (int i = 0; i < (int)(x); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); i++)
#define RREP(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define RREPS(i, x) for (int i = ((int)(x)); i > 0; i--)
#define INF 2147483647
#define LLINF 9223372036854775807LL
#define vi vector<int>
#define vvi vector<vector<int>>
#define pi pair<int, int>
#define ALL(a) (a).begin(), (a).end()
#define BIT(n) (1LL << (n))
#define UNIQUE(v)                                                              \
  v.erase(unique(v.begin(), v.end()), v.end()) // sortしてからつかうこと
constexpr ll MOD = 1e9 + 7;
int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
int dy[8] = {0, 1, 0, -1, 1, -1, 1, -1};
ll A, B, C, D, E, F, G, H, N, M, L, K, P, Q, R, W, X, Y, Z;
string S, T;
ll ans = 0;

template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}

int intceil(int a, int b) { return (a + (b - 1)) / b; }

class Fp {
public:
  ll val;
  static ll mod;

  Fp() { val = 0; }
  Fp(ll v) : val(v % mod) {
    if (val < 0)
      val += mod;
  }
  Fp operator+=(const Fp &in) {
    val += in.val;
    calc_mod(val);
    return *this;
  }
  Fp operator+(const Fp &in) const { return Fp(*this) += in; }
  Fp &operator-=(const Fp &in) {
    val -= in.val;
    calc_mod(val);
    if (val < 0)
      val += mod;
    return *this;
  }
  Fp operator-(const Fp &in) const { return Fp(*this) -= in; }
  Fp &operator*=(const Fp &in) {
    val = val * in.val % mod;
    return *this;
  }
  Fp operator*(const Fp &in) const { return Fp(*this) *= in; }
  Fp &operator/=(const Fp &in) {
    *this *= in.modpow(mod - 2);
    return *this;
  }
  Fp operator/(const Fp &in) const { return Fp(*this) /= in; }
  bool operator==(const Fp &in) const { return val == in.val; }
  bool operator!=(const Fp &in) const { return val != in.val; }
  Fp modpow(ll n) const {
    if (n == 0)
      return 1;
    if (n & 1) {
      return (*this) * ((*this) * (*this)).modpow(n / 2);
    } else {
      return ((*this) * (*this)).modpow(n / 2);
    }
  }
  friend istream &operator>>(istream &is, Fp &fp) {
    is >> fp.val;
    return is;
  }
  friend ostream &operator<<(ostream &os, Fp &fp) {
    os << fp.val;
    return os;
  }
  friend ostream &operator<<(ostream &os, Fp &&fp) {
    os << fp.val;
    return os;
  }

private:
  void calc_mod(ll &v) {
    if (v >= mod)
      v -= mod;
  }
};

Fp modpow(const Fp &in, const ll n) { return in.modpow(n); }
Fp modpow(const ll a, const ll n) { return Fp(a).modpow(n); }

using mint = Fp;
ll mint::mod = MOD;

class Factorial {
public:
  vector<mint> fact, finv;
  Factorial(ll n) : fact(n + 1, mint(1)), finv(n + 1, mint(1)) {
    for (int i = 2; i <= n; i++) {
      mint tmp(i);
      fact[i] = fact[i - 1] * tmp;
      finv[i] = finv[i - 1] / tmp;
    }
  }
  mint perm(const ll &n, const ll &k) { return fact[n] * finv[n - k]; }
  mint com(const ll &n, const ll &k) {
    if (n < k || n < 0 || k < 0)
      return mint(0);
    return fact[n] * finv[k] * finv[n - k];
  }
};

signed main() {
  cin >> N;
  cin >> S >> T;
  vi v;
  REP(i, N - 1) {
    if (S[i] == S[i + 1]) {
      v.push_back(2);
      i++;
    } else {
      v.push_back(1);
    }
  }
  if (v.back() == 1) {
    v.push_back(1);
  }
  dump(v);
  mint ans = (v[0] == 1) ? 3 : 6;
  REP(i, v.size()) {
    if (i == 0)
      continue;
    if (v[i] == 2 && v[i - 1] == 2) {
      ans *= 3;
    } else if (v[i] == 1 && v[i - 1] == 2) {
      ans *= 1;
    } else if (v[i] == 2 && v[i - 1] == 1) {
      ans *= 2;
    } else {
      ans *= 2;
    }
  }
  cout << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define dump(...)
#endif
#define endl "\n"
#define ll long long
#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define REP(i, x) for (int i = 0; i < (int)(x); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); i++)
#define RREP(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define RREPS(i, x) for (int i = ((int)(x)); i > 0; i--)
#define INF 2147483647
#define LLINF 9223372036854775807LL
#define vi vector<int>
#define vvi vector<vector<int>>
#define pi pair<int, int>
#define ALL(a) (a).begin(), (a).end()
#define BIT(n) (1LL << (n))
#define UNIQUE(v)                                                              \
  v.erase(unique(v.begin(), v.end()), v.end()) // sortしてからつかうこと
constexpr ll MOD = 1e9 + 7;
int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
int dy[8] = {0, 1, 0, -1, 1, -1, 1, -1};
ll A, B, C, D, E, F, G, H, N, M, L, K, P, Q, R, W, X, Y, Z;
string S, T;
ll ans = 0;

template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}

int intceil(int a, int b) { return (a + (b - 1)) / b; }

class Fp {
public:
  ll val;
  static ll mod;

  Fp() { val = 0; }
  Fp(ll v) : val(v % mod) {
    if (val < 0)
      val += mod;
  }
  Fp operator+=(const Fp &in) {
    val += in.val;
    calc_mod(val);
    return *this;
  }
  Fp operator+(const Fp &in) const { return Fp(*this) += in; }
  Fp &operator-=(const Fp &in) {
    val -= in.val;
    calc_mod(val);
    if (val < 0)
      val += mod;
    return *this;
  }
  Fp operator-(const Fp &in) const { return Fp(*this) -= in; }
  Fp &operator*=(const Fp &in) {
    val = val * in.val % mod;
    return *this;
  }
  Fp operator*(const Fp &in) const { return Fp(*this) *= in; }
  Fp &operator/=(const Fp &in) {
    *this *= in.modpow(mod - 2);
    return *this;
  }
  Fp operator/(const Fp &in) const { return Fp(*this) /= in; }
  bool operator==(const Fp &in) const { return val == in.val; }
  bool operator!=(const Fp &in) const { return val != in.val; }
  Fp modpow(ll n) const {
    if (n == 0)
      return 1;
    if (n & 1) {
      return (*this) * ((*this) * (*this)).modpow(n / 2);
    } else {
      return ((*this) * (*this)).modpow(n / 2);
    }
  }
  friend istream &operator>>(istream &is, Fp &fp) {
    is >> fp.val;
    return is;
  }
  friend ostream &operator<<(ostream &os, Fp &fp) {
    os << fp.val;
    return os;
  }
  friend ostream &operator<<(ostream &os, Fp &&fp) {
    os << fp.val;
    return os;
  }

private:
  void calc_mod(ll &v) {
    if (v >= mod)
      v -= mod;
  }
};

Fp modpow(const Fp &in, const ll n) { return in.modpow(n); }
Fp modpow(const ll a, const ll n) { return Fp(a).modpow(n); }

using mint = Fp;
ll mint::mod = MOD;

class Factorial {
public:
  vector<mint> fact, finv;
  Factorial(ll n) : fact(n + 1, mint(1)), finv(n + 1, mint(1)) {
    for (int i = 2; i <= n; i++) {
      mint tmp(i);
      fact[i] = fact[i - 1] * tmp;
      finv[i] = finv[i - 1] / tmp;
    }
  }
  mint perm(const ll &n, const ll &k) { return fact[n] * finv[n - k]; }
  mint com(const ll &n, const ll &k) {
    if (n < k || n < 0 || k < 0)
      return mint(0);
    return fact[n] * finv[k] * finv[n - k];
  }
};

signed main() {
  cin >> N;
  cin >> S >> T;
  if (N == 1) {
    cout << 3 << endl;
    return 0;
  }
  vi v;
  REP(i, N - 1) {
    if (S[i] == S[i + 1]) {
      v.push_back(2);
      i++;
    } else {
      v.push_back(1);
    }
  }
  if (v.back() == 1) {
    v.push_back(1);
  }
  dump(v);
  mint ans = (v[0] == 1) ? 3 : 6;
  REP(i, v.size()) {
    if (i == 0)
      continue;
    if (v[i] == 2 && v[i - 1] == 2) {
      ans *= 3;
    } else if (v[i] == 1 && v[i - 1] == 2) {
      ans *= 1;
    } else if (v[i] == 2 && v[i - 1] == 1) {
      ans *= 2;
    } else {
      ans *= 2;
    }
  }
  cout << ans << endl;
}