Problem p03003 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cassert>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#define rep(i, n) for (int i = 0; i < n; ++i)
using namespace std;
using ll = long long;
using P = pair<int, int>;
// auto mod int
const int mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

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

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


Fixed submission:
#include <algorithm>
#include <cassert>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#define rep(i, n) for (int i = 0; i < n; ++i)
using namespace std;
using ll = long long;
using P = pair<int, int>;
// auto mod int
const int mod = 1000000007;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

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

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