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

// #define _GLIBCXX_DEBUG
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
#define rep0(TMS) for (int CNT = 0; CNT < (int)(TMS); CNT++)
#define rep(CNT, GOAL) for (int CNT = 0; CNT < (int)(GOAL); CNT++)
#define rep2(CNT, START, GOAL)                                                 \\
  for (int CNT = (int)(START); CNT < (int)(GOAL); CNT++)
#define rep3(CNT, START, GOAL)                                                 \\
  for (int CNT = (int)(START); CNT > (int)(GOAL); CNT--)
#define all(CONT) begin(CONT), end(CONT)
#define itrep(ITR, CONT) for (auto ITR = begin(CONT); ITR != end(CONT); ITR++)
#define itrep1(ITR, CONT)                                                      \\
  for (auto ITR = next(begin(CONT)); ITR != end(CONT); ITR++)
template <typename T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
void prec(const int &DIG) {
  cerr << fixed << setprecision(DIG);
  cout << fixed << setprecision(DIG);
}
template <typename T> void CERR(const T &ELEM) { cerr << ELEM; }
template <typename T, typename... Ts>
void CERR(const T &FIRST, const Ts &...REST) {
  CERR(FIRST);
  cerr << ", ";
  CERR(REST...);
}
template <typename T1, typename T2> void CERR(const pair<T1, T2> &PAIR) {
  cerr << "(";
  CERR(PAIR.first);
  cerr << ", ";
  CERR(PAIR.second);
  cerr << ")";
}
template <typename T> void CERR(const vector<T> &VEC) {
  cerr << "{ ";
  itrep(ITR, VEC) {
    CERR(*ITR);
    cerr << ", ";
  }
  cerr << "}";
}
template <typename T> void CERR1(const vector<T> &VEC) {
  cerr << "{ ";
  itrep1(ITR, VEC) {
    CERR(*ITR);
    cerr << ", ";
  }
  cerr << "}";
}
template <typename T> void CERR(const set<T> &SET) {
  cerr << "{ ";
  itrep(ITR, SET) {
    CERR(*ITR);
    cerr << ", ";
  }
  cerr << "}";
}
template <typename T> void CERR(const multiset<T> &MULTISET) {
  cerr << "{ ";
  itrep(ITR, MULTISET) {
    CERR(*ITR);
    cerr << ", ";
  }
  cerr << "}";
}
template <typename T1, typename T2> void CERR(const map<T1, T2> &MAP) {
  cerr << "{ ";
  itrep(ITR, MAP) {
    CERR(*ITR);
    cerr << ", ";
  }
  cerr << "}";
}
#define db(OBJ)                                                                \\
  cerr << #OBJ << ": ";                                                        \\
  CERR(OBJ);                                                                   \\
  cerr << ", "
#define dbl(OBJ)                                                               \\
  cerr << #OBJ << ": ";                                                        \\
  CERR(OBJ);                                                                   \\
  cerr << "\\n"
#define db1(OBJ)                                                               \\
  cerr << #OBJ << ": ";                                                        \\
  CERR1(OBJ);                                                                  \\
  cerr << "\\n"
#define dbs(...)                                                               \\
  cerr << "(" << #__VA_ARGS__ << "): (";                                       \\
  CERR(__VA_ARGS__);                                                           \\
  cerr << ")\\n"
#define dbvv(VV)                                                               \\
  cerr << #VV << ": {\\n";                                                      \\
  rep(CNT, VV.size()) {                                                        \\
    cerr << #VV << "[" << CNT << "]: ";                                        \\
    CERR(VV[CNT]);                                                             \\
    cerr << ",\\n";                                                             \\
  }                                                                            \\
  cerr << "}\\n"
#define db01(VV)                                                               \\
  cerr << #VV << ": {\\n";                                                      \\
  rep(CNT, VV.size()) {                                                        \\
    cerr << #VV << "[" << CNT << "]: ";                                        \\
    CERR1(VV[CNT]);                                                            \\
    cerr << ",\\n";                                                             \\
  }                                                                            \\
  cerr << "}\\n"
#define db10(VV)                                                               \\
  cerr << #VV << ": {\\n";                                                      \\
  rep2(CNT, 1, VV.size()) {                                                    \\
    cerr << #VV << "[" << CNT << "]: ";                                        \\
    CERR(VV[CNT]);                                                             \\
    cerr << ",\\n";                                                             \\
  }                                                                            \\
  cerr << "}\\n"
#define db11(VV)                                                               \\
  cerr << #VV << ": {\\n";                                                      \\
  rep2(CNT, 1, VV.size()) {                                                    \\
    cerr << #VV << "[" << CNT << "]: ";                                        \\
    CERR1(VV[CNT]);                                                            \\
    cerr << ",\\n";                                                             \\
  }                                                                            \\
  cerr << "}\\n"
#define YN(FLG) cout << (FLG ? "YES" : "NO") << "\\n"
#define Yn(FLG) cout << (FLG ? "Yes" : "No") << "\\n"
#define yn(FLG) cout << (FLG ? "yes" : "no") << "\\n"
// const ll INF = 1'000'000'000'000'000'007;
const int INF = 1'000'000'007;
const ll MOD = 1'000'000'007; // 998'244'353;

ll rem(ll x) {
  x %= MOD;
  if (x < 0)
    x += MOD;
  return x;
}
ll madd(ll x, ll y) { return (rem(x) + rem(y)) % MOD; }
ll msub(ll x, ll y) { return madd(x, -y); }
ll mmul(ll x, ll y) { return rem(x) * rem(y) % MOD; }
ll mpow(ll x, ll n) {
  assert(n >= 0);
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = mmul(res, x);
    x = mmul(x, x);
    n >>= 1;
  }
  return res;
}
ll inv(ll x) {
  assert(x % MOD != 0);
  return mpow(x, MOD - 2);
}
ll mdiv(ll x, ll y) { return mmul(x, inv(y)); }
void mad(ll &x, const ll y) { x = madd(x, y); }
void msb(ll &x, const ll y) { x = msub(x, y); }
void mml(ll &x, const ll y) { x = mmul(x, y); }
void mdv(ll &x, const ll y) { x = mdiv(x, y); }

int main() {
  int N;
  cin >> N;
  vl A(N);
  rep(i, N) cin >> A[i];
  ll sum = 0;
  rep(i, N) sum = (sum + A[i]) % MOD; // Aiの総和
  // cout << "sum:" << sum << endl;
  ll qsum = 0;
  rep(i, N) qsum = (qsum + (A[i] * A[i]) % MOD) % MOD; // Ai^2の総和
  // cout << "qsum:" << qsum << endl;
  ll ans = (sum * sum) % MOD;
  // cout << "ans:" << ans << endl;
  ans = (ans - qsum) % MOD;
  // cout << "ans:" << ans << endl;
  if (ans < 0)
    ans += MOD;
  // cout << "ans:" << ans << endl;
  ans = (ans * 500'000'004) % MOD;
  // cout << "ans:" << ans << endl;
  cout << ans << endl;
}


Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int MOD = 1e9 + 7;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main() {
  int N;
  cin >> N;
  vector<ll> A(N);
  // for (int i = 0; i < N; i++) cin >> A[i];
  rep(i, N) cin >> A[i];
  ll ans = 0;
  for (int i = 0; i < N - 1; i++) {
    for (int j = i + 1; j < N; j++) {
      ans = (ans + A[i] * A[j]) % MOD;
    }
  }
  cout << ans << endl;
}