Problem p03172 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#ifdef __LOCAL
#define DBG(X) cout << #X << " = " << (X) << endl;
#define SAY(X) cout << (X) << endl;
#else
#define DBG(X)
#define SAY(X)
#endif

#ifdef __LOCAL
#include <filesystem>
namespace fs = std::filesystem;
#endif

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

inline void fast_io() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
};
template <typename T, typename S>
inline ostream &operator<<(ostream &os, const pair<T, S> p) {
  cout << "[" << p.first << ";" << p.second << "]";
  return os;
}
template <typename T, typename S>
inline ostream &operator<<(ostream &os, const map<T, S> p) {
  for (auto el : p)
    os << "[" << el.first << " " << el.second << "]";
  return os;
}
template <typename T>
inline ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto el : v)
    os << el << " ";
  return os;
}
template <typename T> inline vector<T> fetch_vec(int sz) {
  vector<T> ret(sz);
  for (auto &elem : ret)
    cin >> elem;
  return ret;
}

template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt<mod>(t);
    return (is);
  }

  static int get_mod() { return mod; }
};
ll const mod = 1e9 + 7;
using modint = ModInt<mod>;

int N, K;
vector<ll> A;

void input() {
  fast_io();
#ifdef __LOCAL
  fs::path p = __FILE__;
  fs::path input, output;
  input = output = p.parent_path();
  input += string("/input/") + string(p.stem()) + string(".txt");
  output += string("/output/") + string(p.stem()) + string(".txt");
  freopen(input.c_str(), "r", stdin);
  freopen(output.c_str(), "w", stdout);
#endif
  cin >> N >> K;
  for (int i = 0; i < N; i++) {
    ll a;
    cin >> a;
    A.push_back(a);
  }
}

int solve() {

  // modint dp[105][100010];
  vector<vector<modint>> dp(N + 1, vector<modint>(K, 0));
  // memset(dp,0,sizeof(dp));
  dp[0][0] = 1;
  for (int i = 1; i <= N; i++) {
    for (int j = 0; j <= K; j++) {
      dp[i][j] += dp[i - 1][j];
      if (j)
        dp[i][j] += dp[i][j - 1];
      if (j - A[i - 1] - 1 >= 0)
        dp[i][j] -= dp[i - 1][j - A[i - 1] - 1];
    }
  }
  cout << dp[N][K] << endl;

  return 0;
}

int main() {
  input();
  solve();
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#ifdef __LOCAL
#define DBG(X) cout << #X << " = " << (X) << endl;
#define SAY(X) cout << (X) << endl;
#else
#define DBG(X)
#define SAY(X)
#endif

#ifdef __LOCAL
#include <filesystem>
namespace fs = std::filesystem;
#endif

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

inline void fast_io() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
};
template <typename T, typename S>
inline ostream &operator<<(ostream &os, const pair<T, S> p) {
  cout << "[" << p.first << ";" << p.second << "]";
  return os;
}
template <typename T, typename S>
inline ostream &operator<<(ostream &os, const map<T, S> p) {
  for (auto el : p)
    os << "[" << el.first << " " << el.second << "]";
  return os;
}
template <typename T>
inline ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto el : v)
    os << el << " ";
  return os;
}
template <typename T> inline vector<T> fetch_vec(int sz) {
  vector<T> ret(sz);
  for (auto &elem : ret)
    cin >> elem;
  return ret;
}

template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt<mod>(t);
    return (is);
  }

  static int get_mod() { return mod; }
};
ll const mod = 1e9 + 7;
using modint = ModInt<mod>;

int N, K;
vector<ll> A;

void input() {
  fast_io();
#ifdef __LOCAL
  fs::path p = __FILE__;
  fs::path input, output;
  input = output = p.parent_path();
  input += string("/input/") + string(p.stem()) + string(".txt");
  output += string("/output/") + string(p.stem()) + string(".txt");
  freopen(input.c_str(), "r", stdin);
  freopen(output.c_str(), "w", stdout);
#endif
  cin >> N >> K;
  for (int i = 0; i < N; i++) {
    ll a;
    cin >> a;
    A.push_back(a);
  }
}

int solve() {

  // modint dp[105][100010];
  vector<vector<modint>> dp(N + 10, vector<modint>(K + 10, 0));
  // memset(dp,0,sizeof(dp));
  dp[0][0] = 1;
  for (int i = 1; i <= N; i++) {
    for (int j = 0; j <= K; j++) {
      dp[i][j] += dp[i - 1][j];
      if (j)
        dp[i][j] += dp[i][j - 1];
      if (j - A[i - 1] - 1 >= 0)
        dp[i][j] -= dp[i - 1][j - A[i - 1] - 1];
    }
  }
  cout << dp[N][K] << endl;

  return 0;
}

int main() {
  input();
  solve();
  return 0;
}