Problem p03172 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x, vector<x>, greater<x>>
#define all(x) (x).begin(), (x).end()
#define CST(x) cout << fixed << setprecision(x)
#define rev(x) reverse(x);
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
using vpl = vector<pl>;
using vvpl = vector<vpl>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[9] = {1, 0, -1, 0, 1, 1, -1, -1, 0};
const ll dx[9] = {0, -1, 0, 1, 1, -1, 1, -1, 0};
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

const int mod = 1000000007;
const int max_n = 200005;
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, mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

int main() {
  ll n, k;
  cin >> n >> k;
  vl a(n);
  rep(i, n) cin >> a[i];
  vector<vector<mint>> dp(n + 1, vector<mint>(k + 1)),
      cum(n + 1, vector<mint>(k + 2));
  dp[0][0] = 1;
  rep(i, k + 2) cum[0][i] = 1;
  cum[0][0] = 0;
  rep(i, n) {
    rep(j, k + 1) {
      dp[i + 1][j] += cum[i][j + 1];
      if (j - a[i] >= 0)
        dp[i + 1][j] -= cum[i][j - a[i]];
    }
    rep(j, k + 2) cum[i + 1][j + 1] += cum[i + 1][j] + dp[i + 1][j];
  }
  /*rep(i,n+1){
      rep(j,k+1){
          cout << dp[i][j] <<" ";
      }
      cout << endl;
  }
  cout << endl;
  rep(i,n+1){
      rep(j,k+2){
          cout << cum[i][j] <<" ";
      }
      cout << endl;
  }*/
  cout << dp[n][k] << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x, vector<x>, greater<x>>
#define all(x) (x).begin(), (x).end()
#define CST(x) cout << fixed << setprecision(x)
#define rev(x) reverse(x);
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
using vpl = vector<pl>;
using vvpl = vector<vpl>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[9] = {1, 0, -1, 0, 1, 1, -1, -1, 0};
const ll dx[9] = {0, -1, 0, 1, 1, -1, 1, -1, 0};
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

const int mod = 1000000007;
const int max_n = 200005;
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, mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

int main() {
  ll n, k;
  cin >> n >> k;
  vl a(n);
  rep(i, n) cin >> a[i];
  vector<vector<mint>> dp(n + 1, vector<mint>(k + 1)),
      cum(n + 1, vector<mint>(k + 2));
  dp[0][0] = 1;
  rep(i, k + 2) cum[0][i] = 1;
  cum[0][0] = 0;
  rep(i, n) {
    rep(j, k + 1) {
      dp[i + 1][j] += cum[i][j + 1];
      if (j - a[i] >= 0)
        dp[i + 1][j] -= cum[i][j - a[i]];
    }
    rep(j, k + 1) cum[i + 1][j + 1] += cum[i + 1][j] + dp[i + 1][j];
  }
  /*rep(i,n+1){
      rep(j,k+1){
          cout << dp[i][j] <<" ";
      }
      cout << endl;
  }
  cout << endl;
  rep(i,n+1){
      rep(j,k+2){
          cout << cum[i][j] <<" ";
      }
      cout << endl;
  }*/
  cout << dp[n][k] << endl;
}