Problem p03003 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>

// #undef DEBUG  // Uncomment this line to forcefully disable debug print.
#if DEBUG
template <typename T> void debug(T value) { std::cerr << value; }
template <typename T, typename... Ts> void debug(T value, Ts... args) {
  std::cerr << value << ", ";
  debug(args...);
}
#define dbg(...)                                                               \
  do {                                                                         \
    cerr << #__VA_ARGS__ << ": ";                                              \
    debug(__VA_ARGS__);                                                        \
    cerr << " (L" << __LINE__ << ")" << endl;                                  \
  } while (0)
#else
#define dbg(...)
#endif

void read_from_cin() {}
template <typename T, typename... Ts>
void read_from_cin(T &value, Ts &...args) {
  std::cin >> value;
  read_from_cin(args...);
}
#define in(type, ...)                                                          \
  type __VA_ARGS__;                                                            \
  read_from_cin(__VA_ARGS__);

template <typename T> void write_to_cout(const T &value) {
  std::cout << value << std::endl;
}
template <typename T, typename... Ts>
void write_to_cout(const T &value, const Ts &...args) {
  std::cout << value << ' ';
  write_to_cout(args...);
}
#define out(...) write_to_cout(__VA_ARGS__);

#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)

using ll = long long;

using namespace std;

template <int Mod> class ModInt {
public:
  ModInt() : n_(0) {}
  ModInt(long long n) : n_(n % Mod) {
    if (n_ < 0) {
      // In C++, (-n)%m == -(n%m).
      n_ += Mod;
    }
  }
  ModInt &operator+=(const ModInt &m) {
    n_ += m.n_;
    if (n_ >= Mod) {
      n_ -= Mod;
    }
    return *this;
  }
  ModInt &operator++() { return (*this) += 1; }
  ModInt &operator-=(const ModInt &m) {
    n_ -= m.n_;
    if (n_ < 0) {
      n_ += Mod;
    }
    return *this;
  }
  ModInt &operator--() { return (*this) -= 1; }
  ModInt &operator*=(const ModInt &m) {
    n_ *= m.n_;
    n_ %= Mod;
    return *this;
  }
  ModInt &operator/=(const ModInt &m) {
    *this *= m.Inv();
    return *this;
  }
#define DEFINE_BINARY_OPERATOR(op)                                             \
  ModInt operator op(const ModInt &m) const { return ModInt(*this) op## = m; }
  DEFINE_BINARY_OPERATOR(+)
  DEFINE_BINARY_OPERATOR(-)
  DEFINE_BINARY_OPERATOR(*)
  DEFINE_BINARY_OPERATOR(/)
#undef DEFINE_BINARY_OPERATOR
#define DEFINE_COMPARISON_OPERATOR(op)                                         \
  bool operator op(const ModInt &m) const { return n_ op m.n_; }
  DEFINE_COMPARISON_OPERATOR(!=)
  DEFINE_COMPARISON_OPERATOR(<)
  DEFINE_COMPARISON_OPERATOR(<=)
  DEFINE_COMPARISON_OPERATOR(==)
  DEFINE_COMPARISON_OPERATOR(>)
  DEFINE_COMPARISON_OPERATOR(>=)
#undef BDEFINE_COMPARISON_OPERATOR
  ModInt operator-() const { return ModInt(-n_); }
  ModInt Pow(int n) const {
    if (n < 0) {
      return Inv().Pow(-n);
    }
    // a * b ^ n = answer.
    ModInt a = 1, b = *this;
    while (n != 0) {
      if (n & 1) {
        a *= b;
      }
      n /= 2;
      b *= b;
    }
    return a;
  }
  ModInt Inv() const {
    // Compute the inverse based on Fermat's little theorem. Note that this only
    // works when n_ and Mod are relatively prime. The theorem says that
    // n_^(Mod-1) = 1 (mod Mod). So we can compute n_^(Mod-2).
    return Pow(Mod - 2);
  }
  long long value() const { return n_; }

  static ModInt Fact(int n) {
    for (int i = fact_.size(); i <= n; ++i) {
      fact_.push_back(i == 0 ? 1 : fact_.back() * i);
    }
    return fact_[n];
  }
  static ModInt Comb(int n, int k) { return Perm(n, k) / Fact(k); }
  static ModInt CombSlow(int n, int k) { return PermSlow(n, k) / Fact(k); }
  static ModInt Perm(int n, int k) {
#if DEBUG
    assert(n <= 1000000 &&
           "n is too large. If k is small, consider using PermSlow.");
#endif
    return Fact(n) / Fact(n - k);
  }
  static ModInt PermSlow(int n, int k) {
    ModInt p = 1;
    for (int i = 0; i < k; ++i) {
      p *= (n - i);
    }
    return p;
  }

private:
  long long n_;
  static std::vector<ModInt> fact_;
};

template <int Mod> std::vector<ModInt<Mod>> ModInt<Mod>::fact_;

template <int Mod>
std::ostream &operator<<(std::ostream &out, const ModInt<Mod> &m) {
  out << m.value();
  return out;
}

using mint = ModInt<1000000007>;

int main() {
  in(int, n, m);
  vector<int> s(n);
  rep(i, n) cin >> s[i];
  vector<int> t(m);
  rep(i, m) cin >> t[i];

  vector<vector<mint>> dp(n + 1, vector<mint>(m + 1));
  rep(i, n + 1) dp[i][0] = 1;
  rep(i, m + 1) dp[0][i] = 1;
  rep(i, n) rep(j, m) {
    dp[i + 1][j + 1] = dp[i + 1][j] + dp[i][j + 1] - dp[i][j];
    if (s[i] == t[j])
      dp[i + 1][j + 1] += dp[i][j];
  }
  rep(i, n + 1) {
    rep(j, m + 1) cerr << dp[i][j] << " ";
    cerr << endl;
  }
  out(dp[n][m]);
}


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

// #undef DEBUG  // Uncomment this line to forcefully disable debug print.
#if DEBUG
template <typename T> void debug(T value) { std::cerr << value; }
template <typename T, typename... Ts> void debug(T value, Ts... args) {
  std::cerr << value << ", ";
  debug(args...);
}
#define dbg(...)                                                               \
  do {                                                                         \
    cerr << #__VA_ARGS__ << ": ";                                              \
    debug(__VA_ARGS__);                                                        \
    cerr << " (L" << __LINE__ << ")" << endl;                                  \
  } while (0)
#else
#define dbg(...)
#endif

void read_from_cin() {}
template <typename T, typename... Ts>
void read_from_cin(T &value, Ts &...args) {
  std::cin >> value;
  read_from_cin(args...);
}
#define in(type, ...)                                                          \
  type __VA_ARGS__;                                                            \
  read_from_cin(__VA_ARGS__);

template <typename T> void write_to_cout(const T &value) {
  std::cout << value << std::endl;
}
template <typename T, typename... Ts>
void write_to_cout(const T &value, const Ts &...args) {
  std::cout << value << ' ';
  write_to_cout(args...);
}
#define out(...) write_to_cout(__VA_ARGS__);

#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)

using ll = long long;

using namespace std;

template <int Mod> class ModInt {
public:
  ModInt() : n_(0) {}
  ModInt(long long n) : n_(n % Mod) {
    if (n_ < 0) {
      // In C++, (-n)%m == -(n%m).
      n_ += Mod;
    }
  }
  ModInt &operator+=(const ModInt &m) {
    n_ += m.n_;
    if (n_ >= Mod) {
      n_ -= Mod;
    }
    return *this;
  }
  ModInt &operator++() { return (*this) += 1; }
  ModInt &operator-=(const ModInt &m) {
    n_ -= m.n_;
    if (n_ < 0) {
      n_ += Mod;
    }
    return *this;
  }
  ModInt &operator--() { return (*this) -= 1; }
  ModInt &operator*=(const ModInt &m) {
    n_ *= m.n_;
    n_ %= Mod;
    return *this;
  }
  ModInt &operator/=(const ModInt &m) {
    *this *= m.Inv();
    return *this;
  }
#define DEFINE_BINARY_OPERATOR(op)                                             \
  ModInt operator op(const ModInt &m) const { return ModInt(*this) op## = m; }
  DEFINE_BINARY_OPERATOR(+)
  DEFINE_BINARY_OPERATOR(-)
  DEFINE_BINARY_OPERATOR(*)
  DEFINE_BINARY_OPERATOR(/)
#undef DEFINE_BINARY_OPERATOR
#define DEFINE_COMPARISON_OPERATOR(op)                                         \
  bool operator op(const ModInt &m) const { return n_ op m.n_; }
  DEFINE_COMPARISON_OPERATOR(!=)
  DEFINE_COMPARISON_OPERATOR(<)
  DEFINE_COMPARISON_OPERATOR(<=)
  DEFINE_COMPARISON_OPERATOR(==)
  DEFINE_COMPARISON_OPERATOR(>)
  DEFINE_COMPARISON_OPERATOR(>=)
#undef BDEFINE_COMPARISON_OPERATOR
  ModInt operator-() const { return ModInt(-n_); }
  ModInt Pow(int n) const {
    if (n < 0) {
      return Inv().Pow(-n);
    }
    // a * b ^ n = answer.
    ModInt a = 1, b = *this;
    while (n != 0) {
      if (n & 1) {
        a *= b;
      }
      n /= 2;
      b *= b;
    }
    return a;
  }
  ModInt Inv() const {
    // Compute the inverse based on Fermat's little theorem. Note that this only
    // works when n_ and Mod are relatively prime. The theorem says that
    // n_^(Mod-1) = 1 (mod Mod). So we can compute n_^(Mod-2).
    return Pow(Mod - 2);
  }
  long long value() const { return n_; }

  static ModInt Fact(int n) {
    for (int i = fact_.size(); i <= n; ++i) {
      fact_.push_back(i == 0 ? 1 : fact_.back() * i);
    }
    return fact_[n];
  }
  static ModInt Comb(int n, int k) { return Perm(n, k) / Fact(k); }
  static ModInt CombSlow(int n, int k) { return PermSlow(n, k) / Fact(k); }
  static ModInt Perm(int n, int k) {
#if DEBUG
    assert(n <= 1000000 &&
           "n is too large. If k is small, consider using PermSlow.");
#endif
    return Fact(n) / Fact(n - k);
  }
  static ModInt PermSlow(int n, int k) {
    ModInt p = 1;
    for (int i = 0; i < k; ++i) {
      p *= (n - i);
    }
    return p;
  }

private:
  long long n_;
  static std::vector<ModInt> fact_;
};

template <int Mod> std::vector<ModInt<Mod>> ModInt<Mod>::fact_;

template <int Mod>
std::ostream &operator<<(std::ostream &out, const ModInt<Mod> &m) {
  out << m.value();
  return out;
}

using mint = ModInt<1000000007>;

int main() {
  in(int, n, m);
  vector<int> s(n);
  rep(i, n) cin >> s[i];
  vector<int> t(m);
  rep(i, m) cin >> t[i];

  vector<vector<mint>> dp(n + 1, vector<mint>(m + 1));
  rep(i, n + 1) dp[i][0] = 1;
  rep(i, m + 1) dp[0][i] = 1;
  rep(i, n) rep(j, m) {
    dp[i + 1][j + 1] = dp[i + 1][j] + dp[i][j + 1];
    if (s[i] != t[j])
      dp[i + 1][j + 1] -= dp[i][j];
  }
  out(dp[n][m]);
}