Problem p02572 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = (0); i < (n); ++i)
#define ALL(obj) (obj).begin(), (obj).end()
#define DEBUG(val) cout << #val << " : " << val << endl
#define MOD 1000000007
constexpr int mod = 1000000007;
class mint {
public:
  long long x;
  constexpr mint(long long x = 0) : x((x % mod + mod) % mod) {}
  constexpr mint operator-() const { return mint(-x); }
  constexpr mint &operator+=(const mint &a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  constexpr mint &operator-=(const mint &a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  constexpr mint &operator*=(const mint &a) {
    (x *= a.x) %= mod;
    return *this;
  }
  constexpr mint operator+(const mint &a) const {
    mint res(*this);
    return res += a;
  }
  constexpr mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  constexpr mint operator*(const mint &a) const {
    mint res(*this);
    return res *= a;
  }
  constexpr mint pow(long long t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  constexpr mint inv() const { return pow(mod - 2); }
  constexpr mint &operator/=(const mint &a) { return (*this) *= a.inv(); }
  constexpr mint operator/(const mint &a) const {
    mint res(*this);
    return res /= a;
  }
};

istream &operator>>(istream &is, const mint &m) {
  is >> m.x;
  return is;
}

ostream &operator<<(ostream &os, const mint &m) {
  os << m.x;
  return os;
}

mint choose(long long n, long long a) {
  mint x, y;
  x.x = 1;
  y.x = 1;
  for (long long i = (0); i < (a); ++i) {
    x *= n - i;
    y *= i + 1;
  }
  return x / y;
}

int main() {
  int n;
  vector<ll> data;
  cin >> n;
  mint sum;
  sum.x = 0;
  REP(i, n) {
    ll x;
    cin >> x;
    data.push_back(x);
    sum += x;
  }
  mint ans;
  ans.x = 0;
  REP(i, n - 1) {
    sum -= data[i];
    mint tmp;
    tmp.x = data[i];
    tmp *= sum;
    ans += tmp;
  }
  cout << ans << endl;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = (0); i < (n); ++i)
#define ALL(obj) (obj).begin(), (obj).end()
#define DEBUG(val) cout << #val << " : " << val << endl
#define MOD 1000000007
constexpr int mod = 1000000007;
class mint {
public:
  long long x;
  constexpr mint(long long x = 0) : x((x % mod + mod) % mod) {}
  constexpr mint operator-() const { return mint(-x); }
  constexpr mint &operator+=(const mint &a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  constexpr mint &operator-=(const mint &a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  constexpr mint &operator*=(const mint &a) {
    (x *= a.x) %= mod;
    return *this;
  }
  constexpr mint operator+(const mint &a) const {
    mint res(*this);
    return res += a;
  }
  constexpr mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  constexpr mint operator*(const mint &a) const {
    mint res(*this);
    return res *= a;
  }
  constexpr mint pow(long long t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  constexpr mint inv() const { return pow(mod - 2); }
  constexpr mint &operator/=(const mint &a) { return (*this) *= a.inv(); }
  constexpr mint operator/(const mint &a) const {
    mint res(*this);
    return res /= a;
  }
};

istream &operator>>(istream &is, const mint &m) {
  is >> m.x;
  return is;
}

ostream &operator<<(ostream &os, const mint &m) {
  os << m.x;
  return os;
}

mint choose(long long n, long long a) {
  mint x, y;
  x.x = 1;
  y.x = 1;
  for (long long i = (0); i < (a); ++i) {
    x *= n - i;
    y *= i + 1;
  }
  return x / y;
}

int main() {
  int n;
  vector<ll> data;
  cin >> n;
  REP(i, n) {
    ll x;
    cin >> x;
    data.push_back(x);
  }
  mint ans;
  ans.x = 0;
  REP(i, n) {
    FOR(j, i + 1, n) {
      mint tmp;
      tmp.x = data[i];
      tmp *= data[j];
      ans += tmp;
    }
  }
  cout << ans << endl;
}