Problem p02572 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define int int64_t
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++(i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++(i))
#define REP_R(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define REP3R(i, m, n) for (int i = (int)(n)-1; (i) >= (int)(m); --(i))
#define REPP(i, n) for (int i = 1; (i) < (int)(n + 1); ++(i))
#define REPR(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define FORR(i, m, n) for (int i = (m)-1; i >= (n); i--)
#define ALL(v) v.begin(), v.end()
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(), (c).end())
#define SORTR(c) sort((c).begin(), (c).end(), greater<int>());
#define BIT(n) (1LL << (n))
#define PCNT(x) __builtin_popcountll(x)
#define GCD(x, y) __gcd(x, y)
#define LCM(x, y) x / __gcd(x, y) * y
#define PB push_back
#define MP make_pair
#define endl "\n"
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
using namespace std;
using Graph = vector<vector<int>>;
using WeightedGraph = vector<vector<pair<int, int>>>;
typedef long double LD;
typedef vector<int> VI;
typedef vector<vector<int>> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
const int INF = 2000000000;
const long double PI = acosl(-1);
const int MOD = 1000000007;
void print() { cout << endl; }
template <class Head, class... Tail> void print(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail) != 0)
    cout << " ";
  print(forward<Tail>(tail)...);
}
template <class T> void print(vector<T> &vec) {
  for (auto &a : vec) {
    cout << a;
    if (&a != &vec.back())
      cout << " ";
  }
  cout << endl;
}
template <class T> void print(vector<vector<T>> &df) {
  for (auto &vec : df) {
    print(vec);
  }
}

struct mint {
  int x;
  mint(int 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(int t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  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, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

signed main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  cout << fixed << setprecision(10);
  mint ans = 0;

  int N;
  cin >> N;
  vector<int> A(N);
  REP(i, N) { cin >> A[i]; }
  REP(i, N - 1) FOR(j, i + 1, N) {
    mint tmp = A[i] * A[j];
    ans += tmp;
  }
  cout << ans << endl;

  return 0;
}

/*
cd ~/Desktop/Programming/AtCoder/contest/abc177/abc177_c && sh
shellscripts/test_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc177/abc177_c && sh
shellscripts/etest_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc177/abc177_c && sh
shellscripts/submit_cpp.sh
*/


Fixed submission:
#include <bits/stdc++.h>
#define int int64_t
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++(i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++(i))
#define REP_R(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define REP3R(i, m, n) for (int i = (int)(n)-1; (i) >= (int)(m); --(i))
#define REPP(i, n) for (int i = 1; (i) < (int)(n + 1); ++(i))
#define REPR(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define FORR(i, m, n) for (int i = (m)-1; i >= (n); i--)
#define ALL(v) v.begin(), v.end()
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(), (c).end())
#define SORTR(c) sort((c).begin(), (c).end(), greater<int>());
#define BIT(n) (1LL << (n))
#define PCNT(x) __builtin_popcountll(x)
#define GCD(x, y) __gcd(x, y)
#define LCM(x, y) x / __gcd(x, y) * y
#define PB push_back
#define MP make_pair
#define endl "\n"
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
using namespace std;
using Graph = vector<vector<int>>;
using WeightedGraph = vector<vector<pair<int, int>>>;
typedef long double LD;
typedef vector<int> VI;
typedef vector<vector<int>> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
const int INF = 2000000000;
const long double PI = acosl(-1);
const int MOD = 1000000007;
void print() { cout << endl; }
template <class Head, class... Tail> void print(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail) != 0)
    cout << " ";
  print(forward<Tail>(tail)...);
}
template <class T> void print(vector<T> &vec) {
  for (auto &a : vec) {
    cout << a;
    if (&a != &vec.back())
      cout << " ";
  }
  cout << endl;
}
template <class T> void print(vector<vector<T>> &df) {
  for (auto &vec : df) {
    print(vec);
  }
}

struct mint {
  int x;
  mint(int 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(int t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  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, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

signed main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  cout << fixed << setprecision(10);
  mint ans = 0;

  int N;
  cin >> N;
  vector<int> A(N);
  VI B(N + 1, 0);
  REP(i, N) {
    cin >> A[i];
    B[i + 1] = B[i] + A[i];
    B[i + 1] %= MOD;
  }
  REP(i, N) {
    mint tmp = B[N] - B[i + 1];
    tmp *= A[i];
    ans += tmp;
  }
  cout << ans << endl;

  return 0;
}

/*
cd ~/Desktop/Programming/AtCoder/contest/abc177/abc177_c && sh
shellscripts/test_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc177/abc177_c && sh
shellscripts/etest_cpp.sh cd
~/Desktop/Programming/AtCoder/contest/abc177/abc177_c && sh
shellscripts/submit_cpp.sh
*/