Problem p03003 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;

const ll INF64 = 1LL << 60;
const int INF32 = 1 << 29;
const int MOD = 1000000007;

template <int MOD> struct Fp {
  long long val;
  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      v += MOD;
  }
  constexpr int getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    long long a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
    return is >> x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};

using mint = Fp<MOD>;

int main() {
#ifdef MYLOCAL
  ifstream in("input.txt");
  cin.rdbuf(in.rdbuf());
#endif
  int N, M;
  cin >> N >> M;
  vector<int> S(N), T(N);
  for (int i = 0; i < N; ++i)
    cin >> S[i];
  for (int i = 0; i < M; ++i)
    cin >> T[i];

  mint dp[2100][2100];
  memset(dp, 0, sizeof(dp));
  dp[0][0] = 1;
  for (int i = 0; i <= N; ++i) {
    for (int j = 0; j <= M; ++j) {
      if (i - 1 >= 0 && j - 1 >= 0 && S[i - 1] == T[j - 1]) {
        dp[i][j] += dp[i - 1][j - 1];
      }
      if (i - 1 >= 0) {
        dp[i][j] += dp[i - 1][j];
      }
      if (j - 1 >= 0) {
        dp[i][j] += dp[i][j - 1];
      }
      if (i - 1 >= 0 && j - 1 >= 0) {
        dp[i][j] -= dp[i - 1][j - 1];
      }
    }
  }
  cout << dp[N][M] << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;

const ll INF64 = 1LL << 60;
const int INF32 = 1 << 29;
const int MOD = 1000000007;

template <int MOD> struct Fp {
  long long val;
  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      v += MOD;
  }
  constexpr int getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    long long a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
    return is >> x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};

using mint = Fp<MOD>;

int main() {
#ifdef MYLOCAL
  ifstream in("input.txt");
  cin.rdbuf(in.rdbuf());
#endif
  int N, M;
  cin >> N >> M;
  vector<int> S(N), T(M);
  for (int i = 0; i < N; ++i)
    cin >> S[i];
  for (int i = 0; i < M; ++i)
    cin >> T[i];

  mint dp[2100][2100];
  memset(dp, 0, sizeof(dp));
  dp[0][0] = 1;
  for (int i = 0; i <= N; ++i) {
    for (int j = 0; j <= M; ++j) {
      if (i - 1 >= 0 && j - 1 >= 0 && S[i - 1] == T[j - 1]) {
        dp[i][j] += dp[i - 1][j - 1];
      }
      if (i - 1 >= 0) {
        dp[i][j] += dp[i - 1][j];
      }
      if (j - 1 >= 0) {
        dp[i][j] += dp[i][j - 1];
      }
      if (i - 1 >= 0 && j - 1 >= 0) {
        dp[i][j] -= dp[i - 1][j - 1];
      }
    }
  }
  cout << dp[N][M] << endl;
  return 0;
}