Problem p02572 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define int long long
#define ll long long
using ull = unsigned long long;
using namespace std;
#define dump(x)                                                                \
  if (dbg) {                                                                   \
    cerr << #x << " = " << (x) << endl;                                        \
  }
#define overload4(_1, _2, _3, _4, name, ...) name
#define FOR1(n) for (ll i = 0; i < (n); ++i)
#define FOR2(i, n) for (ll i = 0; i < (n); ++i)
#define FOR3(i, a, b) for (ll i = (a); i < (b); ++i)
#define FOR4(i, a, b, c) for (ll i = (a); i < (b); i += (c))
#define FOR(...) overload4(__VA_ARGS__, FOR4, FOR3, FOR2, FOR1)(__VA_ARGS__)
#define FORR(i, a, b) for (int i = (a); i <= (b); ++i)
#define bit(n, k) ((n >> k) & 1) /*nのk bit目*/
namespace mydef {
const int INF = 1ll << 60;
const int MOD = 1e9 + 7;
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  } else
    return 0;
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  } else
    return 0;
}
void Yes(bool flag = true) {
  if (flag)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
void No(bool flag = true) { Yes(!flag); }
void YES(bool flag = true) {
  if (flag)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
void NO(bool flag = true) { YES(!flag); }
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}
bool dbg = false;
} // namespace mydef
using namespace mydef;
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(v) (v).begin(), (v).end()
#define SZ(x) ((int)(x).size())
#define vi vector<int>
#define vvi vector<vector<int>>
#define vp vector<pair<int, int>>
#define vvp vector<vector<pair<int, int>>>
#define pi pair<int, int>
// #define P pair<int, int>
// #define V vector<int>
// #define S set<int>
#define asn ans

template <uint MD> struct ModInt {
  using M = ModInt;
  const static M G;
  uint v;
  ModInt(ll _v = 0) { set_v(_v % MD + MD); }
  M &set_v(uint _v) {
    v = (_v < MD) ? _v : _v - MD;
    return *this;
  }
  explicit operator bool() const { return v != 0; }
  M operator-() const { return M() - *this; }
  M operator+(const M &r) const { return M().set_v(v + r.v); }
  M operator-(const M &r) const { return M().set_v(v + MD - r.v); }
  M operator*(const M &r) const { return M().set_v(ull(v) * r.v % MD); }
  M operator/(const M &r) const { return *this * r.inv(); }
  M &operator+=(const M &r) { return *this = *this + r; }
  M &operator-=(const M &r) { return *this = *this - r; }
  M &operator*=(const M &r) { return *this = *this * r; }
  M &operator/=(const M &r) { return *this = *this / r; }
  bool operator==(const M &r) const { return v == r.v; }
  M pow(ll n) const {
    M x = *this, r = 1;
    while (n) {
      if (n & 1)
        r *= x;
      x *= x;
      n >>= 1;
    }
    return r;
  }
  M inv() const { return pow(MD - 2); }
  /*
  M inv() const {
      long long b = MD, u = 1, v = 0;
      long long a = (*this).v;
      while (b) {
          long long t = a / b;
          a -= t * b;
          swap(a, b);
          u -= t * v;
          swap(u, v);
      }
      u %= MD;
      if (u < 0)
          u += MD;
      return M{u};
  }
  */
  friend ostream &operator<<(ostream &os, const M &r) { return os << r.v; }
  friend istream &operator>>(istream &is, M &r) { return is >> r.v; }
};
using Mint = ModInt<MOD>;

int N;
Mint A[101010];
Mint SUM = 0;

void solve() {}

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);

  cin >> N;
  for (int i = 0; i < N; i++) {
    cin >> A[i];
    SUM += A[i];
  }

  Mint ans = SUM * SUM;
  for (int i = 0; i < N; i++) {
    ans -= A[i] * A[i];
  }
  ans /= 2;
  cout << ans << endl;

  solve();
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#define int long long
#define ll long long
using ull = unsigned long long;
using namespace std;
#define dump(x)                                                                \
  if (dbg) {                                                                   \
    cerr << #x << " = " << (x) << endl;                                        \
  }
#define overload4(_1, _2, _3, _4, name, ...) name
#define FOR1(n) for (ll i = 0; i < (n); ++i)
#define FOR2(i, n) for (ll i = 0; i < (n); ++i)
#define FOR3(i, a, b) for (ll i = (a); i < (b); ++i)
#define FOR4(i, a, b, c) for (ll i = (a); i < (b); i += (c))
#define FOR(...) overload4(__VA_ARGS__, FOR4, FOR3, FOR2, FOR1)(__VA_ARGS__)
#define FORR(i, a, b) for (int i = (a); i <= (b); ++i)
#define bit(n, k) ((n >> k) & 1) /*nのk bit目*/
namespace mydef {
const int INF = 1ll << 60;
const int MOD = 1e9 + 7;
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  } else
    return 0;
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  } else
    return 0;
}
void Yes(bool flag = true) {
  if (flag)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
void No(bool flag = true) { Yes(!flag); }
void YES(bool flag = true) {
  if (flag)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
void NO(bool flag = true) { YES(!flag); }
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}
bool dbg = false;
} // namespace mydef
using namespace mydef;
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(v) (v).begin(), (v).end()
#define SZ(x) ((int)(x).size())
#define vi vector<int>
#define vvi vector<vector<int>>
#define vp vector<pair<int, int>>
#define vvp vector<vector<pair<int, int>>>
#define pi pair<int, int>
// #define P pair<int, int>
// #define V vector<int>
// #define S set<int>
#define asn ans

template <uint MD> struct ModInt {
  using M = ModInt;
  const static M G;
  uint v;
  ModInt(ll _v = 0) { set_v(_v % MD + MD); }
  M &set_v(uint _v) {
    v = (_v < MD) ? _v : _v - MD;
    return *this;
  }
  explicit operator bool() const { return v != 0; }
  M operator-() const { return M() - *this; }
  M operator+(const M &r) const { return M().set_v(v + r.v); }
  M operator-(const M &r) const { return M().set_v(v + MD - r.v); }
  M operator*(const M &r) const { return M().set_v(ull(v) * r.v % MD); }
  M operator/(const M &r) const { return *this * r.inv(); }
  M &operator+=(const M &r) { return *this = *this + r; }
  M &operator-=(const M &r) { return *this = *this - r; }
  M &operator*=(const M &r) { return *this = *this * r; }
  M &operator/=(const M &r) { return *this = *this / r; }
  bool operator==(const M &r) const { return v == r.v; }
  M pow(ll n) const {
    M x = *this, r = 1;
    while (n) {
      if (n & 1)
        r *= x;
      x *= x;
      n >>= 1;
    }
    return r;
  }
  M inv() const { return pow(MD - 2); }
  /*
  M inv() const {
      long long b = MD, u = 1, v = 0;
      long long a = (*this).v;
      while (b) {
          long long t = a / b;
          a -= t * b;
          swap(a, b);
          u -= t * v;
          swap(u, v);
      }
      u %= MD;
      if (u < 0)
          u += MD;
      return M{u};
  }
  */
  friend ostream &operator<<(ostream &os, const M &r) { return os << r.v; }
  friend istream &operator>>(istream &is, M &r) { return is >> r.v; }
};
using Mint = ModInt<MOD>;

int N;
Mint A[201011];
Mint SUM = 0;

void solve() {}

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);

  cin >> N;
  for (int i = 0; i < N; i++) {
    cin >> A[i];
    SUM += A[i];
  }

  Mint ans = SUM * SUM;
  for (int i = 0; i < N; i++) {
    ans -= A[i] * A[i];
  }
  ans /= 2;
  cout << ans << endl;

  solve();
  return 0;
}