Problem p03172 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bits/stdc++.h>
#include <cmath>
#include <cstdio>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <utility>
#include <vector>

#define fir first
#define sec second
#define sz(s) (s).size()
#define pb push_back
#define get(n) scanf("%d", &n);
#define gets(s)                                                                \
  string s;                                                                    \
  cin >> (s);
#define prfi(n) printf("%d", &n);
#define prfd(n) printf("%lf", &n);
#define All(s) (s).begin(), (s).end()
#define rep(i, j) for (int(i) = 0; (i) < (j); (i)++)
#define For(i, j, k) for (int(i) = (j); (i) < (k); (i)++)
#define drep(i, j) for (int(i) = (j); (i) >= 0; (i)--)
#define Ford(i, j, k) for (int(i) = (j); i >= (k); i--)
#define vfor(c, v) for (auto(c) : (v))
#define lpi(n) for (int i = 0; i < (n); i++)
#define lpj(n) for (int j = 0; j < (n); j++)
#define lpz(n) for (int z = 0; z < (n); z++)
#define mem(a, b) memset(a, b, sizeof(a));
#define dump(x) std::cout << #x << " = " << (x) << std::endl;
#define debug(x)                                                               \
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
using ull = unsigned long long int;
using ll = long long;
using ld = long double;
using pii = std::pair<int, int>;
using pll = std::pair<ll, ll>;
using vi = std::vector<int>;
using vvi = std::vector<vi>;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vd = std::vector<double>;
using vvd = std::vector<vd>;
using qi = std::queue<int>;
using vpii = std::vector<std::pair<int, int>>;
using vpll = std::vector<pll>;
using namespace std;

const int Mod = (1e9) + 7;
const int INF = 1e9 + 19;
const ll INFL = 1e18 + 19;
const int dx[] = {-1, 0, 0, 1};
const int dy[] = {0, -1, 1, 0};
const int dx2[] = {-1, -1, -1, 0, 0, 0, 1, 1, 1};
const int dy2[] = {1, 0, -1, 1, 0, -1, 1, 0, -1};
//_____________________________________Templates_________________________________________//

template <class T1, class T2> inline void chmin(T1 &a, T2 b) {
  if (a > b)
    a = b;
}
template <class T1, class T2> inline void chmax(T1 &a, T2 b) {
  if (a < b)
    a = b;
}
template <class T> inline void pri(T a) { cout << a << endl; }
template <class Z> using vec = vector<Z>;
template <class T>
using min_priority_queue = priority_queue<T, vector<T>, greater<T>>;
// mainly use for dynamic prog
template <class T1, class T2> void update(T1 &a, T2 b) {
  a += b;
  if (a > Mod)
    a %= Mod;
}

inline void IN(void) { return; }

template <typename First, typename... Rest>
void IN(First &first, Rest &...rest) {
  cin >> first;
  IN(rest...);
  return;
}

inline void OUT(void) {
  cout << "\n";
  return;
}

template <typename First, typename... Rest>
void OUT(First first, Rest... rest) {
  cout << first << " ";
  OUT(rest...);
  return;
}

bool pairsort(pll pl, pll pr) {
  if (pl.first == pr.first)
    return pl.second > pr.second;
  return pl.first < pr.first;
}

int cntbit(ll a, int n, int j) {
  int res = 0;
  For(i, j, n) {
    if (a >> i & 1) {
      res++;
    }
  }
  return res;
}
vector<int> make_bit(int a) {
  vector<int> res;
  lpi(32) if (a & (1 << i)) res.pb(i);
  return res;
}
bool stdbit(int a, int b) { return a & (1 << b); }
int GCD(int a, int b) {
  if (b > a)
    return GCD(b, a);
  if (a % b == 0)
    return b;
  else
    return GCD(b, a % b);
}
int LCM(int a, int b) { return a * b / GCD(a, b); }
int roundup(int a, int b) {
  if (a % b == 0)
    return a / b;
  else
    return (a + b) / b;
}
int rounddown(int a, int b) {
  if (a % b == 0)
    return a / b;
  else {
    return (a - b) / b;
  }
}
ll pow(ll a, ll n) {
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res *= a;
    a *= a;
    n = n >> 1;
  }
  return res;
}
ll GetDiviserCount(ll N) // 約数の個数
{
  ll res = 1;
  For(i, 2, sqrt(N) + 1) {
    ll cnt = 0;
    while (N % i == 0) {
      cnt++;
      N /= i;
    }
    res *= (cnt + 1);
    if (N == 1)
      break;
  }
  if (N != 1)
    res *= 2;
  return res;
}
vll GetDivisor(ll N) // 約数列挙
{
  vll res;
  for (ll i = 1; i * i <= N; i++) {
    if (N % i == 0) {
      res.pb(i);
      if (i * i != N)
        res.pb(N / i);
    }
  }
  sort(All(res));
  return res;
}

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 {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= 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 {
    mint res(*this);
    return res /= a;
  }
  friend ostream &operator<<(ostream &os, const mint &a);
};
ostream &operator<<(ostream &os, const mint &a) {
  os << a.x;
  return os;
}
struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < Mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

//_____________________　following sorce code_________________________//
const int max_n = 3 * (1e5) + 1;
const int max_m = 83 * (1e5) + 1;

int n, m, k;
ll N;
int h, w;
vvi tree;
string S;
int a, b, c;
vi v;
int ans;

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  IN(n, k);
  vi a(n);
  rep(i, n) IN(a[i]);
  vec<vec<mint>> dp(n + 1, vec<mint>(k + 1));
  vec<vec<mint>> sum(n + 1, vec<mint>(k + 1));
  For(i, max(0, k - a[0]), k + 1) {
    dp[0][i] = 1;
    sum[0][i + 1] = sum[0][i] + dp[0][i];
  }
  For(i, 1, n) {
    rep(j, k + 1) {
      dp[i][j] = sum[i - 1][min(k, j + a[i]) + 1] - sum[i - 1][j];
      sum[i][j + 1] = sum[i][j] + dp[i][j];
    }
  }
  mint ans = dp[n - 1][0];
  cout << ans << endl;
  // for(auto c : ans){cout << c << endl;}
  // cout << fixed << setprecision(15) << ans << endl;
  return 0;
}

Fixed submission:
#include <algorithm>
#include <bits/stdc++.h>
#include <cmath>
#include <cstdio>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <utility>
#include <vector>

#define fir first
#define sec second
#define sz(s) (s).size()
#define pb push_back
#define get(n) scanf("%d", &n);
#define gets(s)                                                                \
  string s;                                                                    \
  cin >> (s);
#define prfi(n) printf("%d", &n);
#define prfd(n) printf("%lf", &n);
#define All(s) (s).begin(), (s).end()
#define rep(i, j) for (int(i) = 0; (i) < (j); (i)++)
#define For(i, j, k) for (int(i) = (j); (i) < (k); (i)++)
#define drep(i, j) for (int(i) = (j); (i) >= 0; (i)--)
#define Ford(i, j, k) for (int(i) = (j); i >= (k); i--)
#define vfor(c, v) for (auto(c) : (v))
#define lpi(n) for (int i = 0; i < (n); i++)
#define lpj(n) for (int j = 0; j < (n); j++)
#define lpz(n) for (int z = 0; z < (n); z++)
#define mem(a, b) memset(a, b, sizeof(a));
#define dump(x) std::cout << #x << " = " << (x) << std::endl;
#define debug(x)                                                               \
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
using ull = unsigned long long int;
using ll = long long;
using ld = long double;
using pii = std::pair<int, int>;
using pll = std::pair<ll, ll>;
using vi = std::vector<int>;
using vvi = std::vector<vi>;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vd = std::vector<double>;
using vvd = std::vector<vd>;
using qi = std::queue<int>;
using vpii = std::vector<std::pair<int, int>>;
using vpll = std::vector<pll>;
using namespace std;

const int Mod = (1e9) + 7;
const int INF = 1e9 + 19;
const ll INFL = 1e18 + 19;
const int dx[] = {-1, 0, 0, 1};
const int dy[] = {0, -1, 1, 0};
const int dx2[] = {-1, -1, -1, 0, 0, 0, 1, 1, 1};
const int dy2[] = {1, 0, -1, 1, 0, -1, 1, 0, -1};
//_____________________________________Templates_________________________________________//

template <class T1, class T2> inline void chmin(T1 &a, T2 b) {
  if (a > b)
    a = b;
}
template <class T1, class T2> inline void chmax(T1 &a, T2 b) {
  if (a < b)
    a = b;
}
template <class T> inline void pri(T a) { cout << a << endl; }
template <class Z> using vec = vector<Z>;
template <class T>
using min_priority_queue = priority_queue<T, vector<T>, greater<T>>;
// mainly use for dynamic prog
template <class T1, class T2> void update(T1 &a, T2 b) {
  a += b;
  if (a > Mod)
    a %= Mod;
}

inline void IN(void) { return; }

template <typename First, typename... Rest>
void IN(First &first, Rest &...rest) {
  cin >> first;
  IN(rest...);
  return;
}

inline void OUT(void) {
  cout << "\n";
  return;
}

template <typename First, typename... Rest>
void OUT(First first, Rest... rest) {
  cout << first << " ";
  OUT(rest...);
  return;
}

bool pairsort(pll pl, pll pr) {
  if (pl.first == pr.first)
    return pl.second > pr.second;
  return pl.first < pr.first;
}

int cntbit(ll a, int n, int j) {
  int res = 0;
  For(i, j, n) {
    if (a >> i & 1) {
      res++;
    }
  }
  return res;
}
vector<int> make_bit(int a) {
  vector<int> res;
  lpi(32) if (a & (1 << i)) res.pb(i);
  return res;
}
bool stdbit(int a, int b) { return a & (1 << b); }
int GCD(int a, int b) {
  if (b > a)
    return GCD(b, a);
  if (a % b == 0)
    return b;
  else
    return GCD(b, a % b);
}
int LCM(int a, int b) { return a * b / GCD(a, b); }
int roundup(int a, int b) {
  if (a % b == 0)
    return a / b;
  else
    return (a + b) / b;
}
int rounddown(int a, int b) {
  if (a % b == 0)
    return a / b;
  else {
    return (a - b) / b;
  }
}
ll pow(ll a, ll n) {
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res *= a;
    a *= a;
    n = n >> 1;
  }
  return res;
}
ll GetDiviserCount(ll N) // 約数の個数
{
  ll res = 1;
  For(i, 2, sqrt(N) + 1) {
    ll cnt = 0;
    while (N % i == 0) {
      cnt++;
      N /= i;
    }
    res *= (cnt + 1);
    if (N == 1)
      break;
  }
  if (N != 1)
    res *= 2;
  return res;
}
vll GetDivisor(ll N) // 約数列挙
{
  vll res;
  for (ll i = 1; i * i <= N; i++) {
    if (N % i == 0) {
      res.pb(i);
      if (i * i != N)
        res.pb(N / i);
    }
  }
  sort(All(res));
  return res;
}

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 {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= 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 {
    mint res(*this);
    return res /= a;
  }
  friend ostream &operator<<(ostream &os, const mint &a);
};
ostream &operator<<(ostream &os, const mint &a) {
  os << a.x;
  return os;
}
struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < Mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

//_____________________　following sorce code_________________________//
const int max_n = 3 * (1e5) + 1;
const int max_m = 83 * (1e5) + 1;

int n, m, k;
ll N;
int h, w;
vvi tree;
string S;
int a, b, c;
vi v;
int ans;

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  IN(n, k);
  vi a(n);
  rep(i, n) IN(a[i]);
  vec<vec<mint>> dp(n + 1, vec<mint>(k + 2));
  vec<vec<mint>> sum(n + 1, vec<mint>(k + 2));
  For(i, max(0, k - a[0]), k + 1) {
    dp[0][i] = 1;
    sum[0][i + 1] = sum[0][i] + dp[0][i];
  }
  For(i, 1, n) {
    rep(j, k + 1) {
      dp[i][j] = sum[i - 1][min(k, j + a[i]) + 1] - sum[i - 1][j];
      sum[i][j + 1] = sum[i][j] + dp[i][j];
    }
  }
  mint ans = dp[n - 1][0];
  cout << ans << endl;
  // for(auto c : ans){cout << c << endl;}
  // cout << fixed << setprecision(15) << ans << endl;
  return 0;
}