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

#include <bits/stdc++.h>
using namespace std;

#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define rep2(i, s, n) for (long long i = (s); i < (long long)(n); i++)
#define repi(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2i(i, s, n) for (int i = (s); i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define chmax(s, t) s = max(s, t);
#define chmin(s, t) s = min(s, t);

#define deg2rad(deg) (((deg) / 360) * 2 * M_PI)
#define rad2deg(rad) (((rad) / 2 / M_PI) * 360)

using ll = long long;
using ld = long double;
using vll = vector<ll>;
using vvll = vector<vll>;
using P = pair<ll, ll>;
using vi = vector<int>;
using vvi = vector<vi>;

const ll INF = (1LL << 60);
const int INFi = (1 << 29);

/*素数判定*/
bool is_prime(ll n) {
  if (n == 1)
    return false;
  for (ll i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}

/*約数列挙*/
vll enum_divisors(ll n) {
  vll l;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      l.push_back(i);
      if (n / i != i)
        l.push_back(n / i);
    }
  }
  sort(all(l));
  return l;
}

/*素因数分解*/
vector<P> prime_factorize(ll n) {
  vector<P> l;
  for (ll i = 2; i * i <= n; i++) {
    if (n % i != 0)
      continue;
    ll e = 0;
    while (n % i == 0) {
      e++;
      n /= i;
    }
    l.push_back({i, e});
  }
  if (n != 1)
    l.push_back({n, 1});
  return l;
}

/*最小公倍数*/
ll lcm(ll a, ll b) { return a * b / __gcd(a, b); }

/*最大公約数*/
ll gcd(ll a, ll b) { return __gcd(a, b); }

/*組み合わせ(Combination)*/
const ll CMAX = 1010000;
const ll CMOD = 1e9 + 7;
ll fac[CMAX], finv[CMAX], inv[CMAX];
// テーブルを作る前処理
void combinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < CMAX; i++) {
    fac[i] = fac[i - 1] * i % CMOD;
    inv[i] = CMOD - inv[CMOD % i] * (CMOD / i) % CMOD;
    finv[i] = finv[i - 1] * inv[i] % CMOD;
  }
}
// 二項係数計算
ll comb(ll n, ll k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % CMOD) % CMOD;
}

/*階乗*/
ll factorial(ll n) {
  const ll FMOD = 1e9 + 7;
  ll ret = n;
  for (ll i = n - 1; i > 0; i--) {
    ret *= i;
    ret %= FMOD;
  }
  return (ret ? ret : 1);
}

int main() {

  ll n;
  cin >> n;
  vll a(n);
  rep(i, n) cin >> a[i];

  ll sum = 0;
  ll ans = 0;
  ll mod = (ll)(1e9 + 7);
  rep(i, n) {
    sum += a[i];
    sum %= mod;
  }

  rep(i, n) {
    sum -= a[i];
    if (sum < 0)
      sum += mod;
    ans += a[i] * sum;
    ans %= mod;
  }

  cout << ans << endl;

  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;

#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define rep2(i, s, n) for (long long i = (s); i < (long long)(n); i++)
#define repi(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2i(i, s, n) for (int i = (s); i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define chmax(s, t) s = max(s, t);
#define chmin(s, t) s = min(s, t);

#define deg2rad(deg) (((deg) / 360) * 2 * M_PI)
#define rad2deg(rad) (((rad) / 2 / M_PI) * 360)

using ll = long long;
using ld = long double;
using vll = vector<ll>;
using vvll = vector<vll>;
using P = pair<ll, ll>;
using vi = vector<int>;
using vvi = vector<vi>;

const ll INF = (1LL << 60);
const int INFi = (1 << 29);

/*素数判定*/
bool is_prime(ll n) {
  if (n == 1)
    return false;
  for (ll i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}

/*約数列挙*/
vll enum_divisors(ll n) {
  vll l;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      l.push_back(i);
      if (n / i != i)
        l.push_back(n / i);
    }
  }
  sort(all(l));
  return l;
}

/*素因数分解*/
vector<P> prime_factorize(ll n) {
  vector<P> l;
  for (ll i = 2; i * i <= n; i++) {
    if (n % i != 0)
      continue;
    ll e = 0;
    while (n % i == 0) {
      e++;
      n /= i;
    }
    l.push_back({i, e});
  }
  if (n != 1)
    l.push_back({n, 1});
  return l;
}

/*最小公倍数*/
ll lcm(ll a, ll b) { return a * b / __gcd(a, b); }

/*最大公約数*/
ll gcd(ll a, ll b) { return __gcd(a, b); }

/*組み合わせ(Combination)*/
const ll CMAX = 1010000;
const ll CMOD = 1e9 + 7;
ll fac[CMAX], finv[CMAX], inv[CMAX];
// テーブルを作る前処理
void combinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < CMAX; i++) {
    fac[i] = fac[i - 1] * i % CMOD;
    inv[i] = CMOD - inv[CMOD % i] * (CMOD / i) % CMOD;
    finv[i] = finv[i - 1] * inv[i] % CMOD;
  }
}
// 二項係数計算
ll comb(ll n, ll k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % CMOD) % CMOD;
}

/*階乗*/
ll factorial(ll n) {
  const ll FMOD = 1e9 + 7;
  ll ret = n;
  for (ll i = n - 1; i > 0; i--) {
    ret *= i;
    ret %= FMOD;
  }
  return (ret ? ret : 1);
}

int main() {

  ll n;
  cin >> n;
  vll a(n);
  rep(i, n) cin >> a[i];

  ll ans = 0;
  rep(i, n - 1) {
    rep2(j, i + 1, n) {
      ans += (a[i] * a[j]) % ((ll)(1e9 + 7));
      ans %= ((ll)(1e9 + 7));
    }
  }

  cout << ans << endl;

  return 0;
}