Problem p02959 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>

#define rep(i, n) for (int i = 0; i < (n); i++)
#define EPS (1e-7)
#define INF (2e9)

using namespace std;
typedef long long int ll;
typedef pair<int, int> PI;
typedef pair<ll, ll> PL;
typedef priority_queue<int> PQI;
typedef priority_queue<int, vector<int>, greater<int>> PQSI;
typedef priority_queue<ll> PQL;
typedef priority_queue<ll, vector<ll>, greater<ll>> PQSL;

const int MOD = 1000000007;
struct mint {
  int n;
  mint(int n_ = 0) : n(n_) {}
};
mint operator-(mint a) { return -a.n + MOD * (a.n != 0); }
mint operator+(mint a, mint b) {
  int x = a.n + b.n;
  return x - (x >= MOD) * MOD;
}
mint operator-(mint a, mint b) {
  int x = a.n - b.n;
  return x + (x < 0) * MOD;
}
mint operator*(mint a, mint b) { return (long long)a.n * b.n % MOD; }
mint &operator+=(mint &a, mint b) { return a = a + b; }
mint &operator-=(mint &a, mint b) { return a = a - b; }
mint &operator*=(mint &a, mint b) { return a = a * b; }
istream &operator>>(istream &i, mint &a) { return i >> a.n; }
ostream &operator<<(ostream &o, mint a) { return o << a.n; }

vector<mint> F_{1, 1}, R_{1, 1}, I_{0, 1};
void check_fact(int n) {
  for (int i = I_.size(); i <= n; i++) {
    I_.push_back(I_[MOD % i] * (MOD - MOD / i));
    F_.push_back(F_[i - 1] * i);
    R_.push_back(R_[i - 1] * I_[i]);
  }
}

mint I(int n) {
  check_fact(abs(n));
  return n >= 0 ? I_[n] : -I_[n];
}
mint F(int n) {
  check_fact(n);
  return n < 0 ? 0 : F_[n];
}
mint R(int n) {
  check_fact(n);
  return n < 0 ? 0 : R_[n];
}
mint C(int n, int r) { return F(n) * R(n - r) * R(r); }
mint P(int n, int r) { return F(n) * R(n - r); }
mint H(int n, int r) { return n == 0 ? (r == 0) : C(n + r - 1, r); }

template <typename T> T gcd(T x, T y) {
  if (y == 0)
    return x;
  else
    return gcd(y, x % y);
}

template <typename T> T lcm(T x, T y) { return (x / gcd(x, y)) * y; }

int N;
ll A[100000], B[100000];
int main(void) {
  cin >> N;
  for (int i = 0; i < N + 1; i++)
    scanf("%lld", &A[i]);
  for (int i = 0; i < N; i++)
    scanf("%lld", &B[i]);

  ll monster = 0;
  rep(i, N) {
    if (A[i] < B[i]) {
      if (A[i] + A[i + 1] >= B[i]) {
        A[i + 1] = A[i + 1] - (B[i] - A[i]);
        monster += B[i];
      } else {
        monster += A[i] + A[i + 1];
        A[i + 1] = 0;
      }
    } else {
      monster += B[i];
    }
  }
  cout << monster << endl;
  return 0;
}


Fixed submission:
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>

#define rep(i, n) for (int i = 0; i < (n); i++)
#define EPS (1e-7)
#define INF (2e9)

using namespace std;
typedef long long int ll;
typedef pair<int, int> PI;
typedef pair<ll, ll> PL;
typedef priority_queue<int> PQI;
typedef priority_queue<int, vector<int>, greater<int>> PQSI;
typedef priority_queue<ll> PQL;
typedef priority_queue<ll, vector<ll>, greater<ll>> PQSL;

const int MOD = 1000000007;
struct mint {
  int n;
  mint(int n_ = 0) : n(n_) {}
};
mint operator-(mint a) { return -a.n + MOD * (a.n != 0); }
mint operator+(mint a, mint b) {
  int x = a.n + b.n;
  return x - (x >= MOD) * MOD;
}
mint operator-(mint a, mint b) {
  int x = a.n - b.n;
  return x + (x < 0) * MOD;
}
mint operator*(mint a, mint b) { return (long long)a.n * b.n % MOD; }
mint &operator+=(mint &a, mint b) { return a = a + b; }
mint &operator-=(mint &a, mint b) { return a = a - b; }
mint &operator*=(mint &a, mint b) { return a = a * b; }
istream &operator>>(istream &i, mint &a) { return i >> a.n; }
ostream &operator<<(ostream &o, mint a) { return o << a.n; }

vector<mint> F_{1, 1}, R_{1, 1}, I_{0, 1};
void check_fact(int n) {
  for (int i = I_.size(); i <= n; i++) {
    I_.push_back(I_[MOD % i] * (MOD - MOD / i));
    F_.push_back(F_[i - 1] * i);
    R_.push_back(R_[i - 1] * I_[i]);
  }
}

mint I(int n) {
  check_fact(abs(n));
  return n >= 0 ? I_[n] : -I_[n];
}
mint F(int n) {
  check_fact(n);
  return n < 0 ? 0 : F_[n];
}
mint R(int n) {
  check_fact(n);
  return n < 0 ? 0 : R_[n];
}
mint C(int n, int r) { return F(n) * R(n - r) * R(r); }
mint P(int n, int r) { return F(n) * R(n - r); }
mint H(int n, int r) { return n == 0 ? (r == 0) : C(n + r - 1, r); }

template <typename T> T gcd(T x, T y) {
  if (y == 0)
    return x;
  else
    return gcd(y, x % y);
}

template <typename T> T lcm(T x, T y) { return (x / gcd(x, y)) * y; }

int N;
ll A[100001], B[100000];
int main(void) {
  cin >> N;
  for (int i = 0; i < N + 1; i++)
    scanf("%lld", &A[i]);
  for (int i = 0; i < N; i++)
    scanf("%lld", &B[i]);

  ll monster = 0;
  rep(i, N) {
    if (A[i] < B[i]) {
      if (A[i] + A[i + 1] >= B[i]) {
        A[i + 1] = A[i + 1] - (B[i] - A[i]);
        monster += B[i];
      } else {
        monster += A[i] + A[i + 1];
        A[i + 1] = 0;
      }
    } else {
      monster += B[i];
    }
  }
  cout << monster << endl;
  return 0;
}