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

#include <bits/stdc++.h>
#define rep(i, n) for (int i = (0); i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define limit(x, l, r) max(l, min(x, r))
#define lims(x, l, r) (x = max(l, min(x, r)))
#define isin(x, l, r) ((l) <= (x) && (x) < (r))
#define show(x) cout << #x << " = " << (x) << endl
#define show2(x, y)                                                            \\
  cout << #x << " = " << (x) << ", " << #y << " = " << (y) << endl
#define show3(x, y, z)                                                         \\
  cout << #x << " = " << (x) << ", " << #y << " = " << (y) << ", " << #z       \\
       << " = " << (z) << endl
#define showv(v)                                                               \\
  rep(i, v.size()) printf("%d%c", v[i], i == v.size() - 1 ? '\\n' : ' ')
#define showv2(v) rep(j, v.size()) showv(v[j])
#define showt(t, n) rep(i, n) printf("%d%c", t[i], i == n - 1 ? '\\n' : ' ')
#define showt2(t, r, c) rep(j, r) showt(t[j], c)
#define showvp(p) rep(i, p.size()) printf("%d %d\\n", p[i].first, p[i].second)
#define printv(v) rep(i, v.size()) printf("%d\\n", v[i])
#define printt(t, n) rep(i, n) printf("%d\\n", t[i])
#define incl(v, x) (find(all(v), x) != v.end())
#define incls(s, c) (s.find(c) != string::npos)
#define lb(a, x) distance((a).begin(), lower_bound(all(a), (x)))
#define ub(a, x) distance((a).begin(), upper_bound(all(a), (x)))
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define bit(n, k) ((n >> k) & 1) // nのk bit目
#define bn(x) ((1 << x) - 1)
#define dup(x, y) (((x) + (y)-1) / (y))
#define newline puts("")
#define uni(x) x.erase(unique(all(x)), x.end())
#define SP << " " <<
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vc = vector<char>;
using vvc = vector<vc>;
using vs = vector<string>;
using vb = vector<bool>;
using vvb = vector<vb>;
using P = pair<int, int>;
using T = tuple<int, int, int>;
using vp = vector<P>;
using vt = vector<T>;
const int mod = 1000000007;
const double EPS = 1e-9;
// const long double EPS = 1e-14;
const int INF = (1 << 30) - 1;
const ll LINF = (1LL << 62) - 1;
#define dame                                                                   \\
  {                                                                            \\
    puts("No");                                                                \\
    return 0;                                                                  \\
  }
#define yn                                                                     \\
  { puts("Yes"); }                                                             \\
  else {                                                                       \\
    puts("No");                                                                \\
  }
#define YN                                                                     \\
  { puts("YES"); }                                                             \\
  else {                                                                       \\
    puts("NO");                                                                \\
  }
inline int in() {
  int x;
  cin >> x;
  return x;
}
inline ll lin() {
  ll x;
  cin >> x;
  return x;
}
inline char chin() {
  char x;
  cin >> x;
  return x;
}
inline string stin() {
  string x;
  cin >> x;
  return x;
}
inline double din() {
  double x;
  cin >> x;
  return x;
}
// template<class T = int> inline T in() { T x; cin >> x; return (x);}
template <typename T> inline ll suma(const vector<T> &a) {
  ll res(0);
  for (auto &&x : a)
    res += x;
  return res;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
char itoa(int n) { return n + '0'; }
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
const int dy[4] = {0, -1, 0, 1};
const int dx[4] = {1, 0, -1, 0};

// const int mod = 1000000007;
//  mint(a).pow(b) -> a^b
struct mint {
  ll x;
  mint(ll x = 0) : x(x % mod) {}
  mint operator-() const { return mint(-x); }
  mint operator~() const { return mint(1) / *this; }
  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 {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
  bool operator<(const mint &a) const { return x < a.x; }
  bool operator==(const mint &a) const { return x == a.x; }
};
istream &operator>>(istream &i, mint &a) {
  i >> a.x;
  return i;
}
ostream &operator<<(ostream &o, const mint &a) {
  o << a.x;
  return o;
}

int main() {
  int n;
  cin >> n;
  vector<mint> a(n);
  rep(i, n) cin >> a[i];

  vector<mint> csum(n + 1);
  rep(i, n) csum[i + 1] = csum[i] + a[i];

  mint ans = 0;
  rep(i, n - 1) ans += a[i] * (csum[n] - csum[i + 1]);
  cout << ans << endl;
  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
#define rep(i, n) for (int i = (0); i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define limit(x, l, r) max(l, min(x, r))
#define lims(x, l, r) (x = max(l, min(x, r)))
#define isin(x, l, r) ((l) <= (x) && (x) < (r))
#define show(x) cout << #x << " = " << (x) << endl
#define show2(x, y)                                                            \\
  cout << #x << " = " << (x) << ", " << #y << " = " << (y) << endl
#define show3(x, y, z)                                                         \\
  cout << #x << " = " << (x) << ", " << #y << " = " << (y) << ", " << #z       \\
       << " = " << (z) << endl
#define showv(v)                                                               \\
  rep(i, v.size()) printf("%d%c", v[i], i == v.size() - 1 ? '\\n' : ' ')
#define showv2(v) rep(j, v.size()) showv(v[j])
#define showt(t, n) rep(i, n) printf("%d%c", t[i], i == n - 1 ? '\\n' : ' ')
#define showt2(t, r, c) rep(j, r) showt(t[j], c)
#define showvp(p) rep(i, p.size()) printf("%d %d\\n", p[i].first, p[i].second)
#define printv(v) rep(i, v.size()) printf("%d\\n", v[i])
#define printt(t, n) rep(i, n) printf("%d\\n", t[i])
#define incl(v, x) (find(all(v), x) != v.end())
#define incls(s, c) (s.find(c) != string::npos)
#define lb(a, x) distance((a).begin(), lower_bound(all(a), (x)))
#define ub(a, x) distance((a).begin(), upper_bound(all(a), (x)))
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define bit(n, k) ((n >> k) & 1) // nのk bit目
#define bn(x) ((1 << x) - 1)
#define dup(x, y) (((x) + (y)-1) / (y))
#define newline puts("")
#define uni(x) x.erase(unique(all(x)), x.end())
#define SP << " " <<
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vc = vector<char>;
using vvc = vector<vc>;
using vs = vector<string>;
using vb = vector<bool>;
using vvb = vector<vb>;
using P = pair<int, int>;
using T = tuple<int, int, int>;
using vp = vector<P>;
using vt = vector<T>;
const int mod = 1000000007;
const double EPS = 1e-9;
// const long double EPS = 1e-14;
const int INF = (1 << 30) - 1;
const ll LINF = (1LL << 62) - 1;
#define dame                                                                   \\
  {                                                                            \\
    puts("No");                                                                \\
    return 0;                                                                  \\
  }
#define yn                                                                     \\
  { puts("Yes"); }                                                             \\
  else {                                                                       \\
    puts("No");                                                                \\
  }
#define YN                                                                     \\
  { puts("YES"); }                                                             \\
  else {                                                                       \\
    puts("NO");                                                                \\
  }
inline int in() {
  int x;
  cin >> x;
  return x;
}
inline ll lin() {
  ll x;
  cin >> x;
  return x;
}
inline char chin() {
  char x;
  cin >> x;
  return x;
}
inline string stin() {
  string x;
  cin >> x;
  return x;
}
inline double din() {
  double x;
  cin >> x;
  return x;
}
// template<class T = int> inline T in() { T x; cin >> x; return (x);}
template <typename T> inline ll suma(const vector<T> &a) {
  ll res(0);
  for (auto &&x : a)
    res += x;
  return res;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
char itoa(int n) { return n + '0'; }
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
const int dy[4] = {0, -1, 0, 1};
const int dx[4] = {1, 0, -1, 0};

// const int mod = 1000000007;
//  mint(a).pow(b) -> a^b
struct mint {
  ll x;
  mint(ll x = 0) : x(x % mod) {}
  mint operator-() const { return mint(-x); }
  mint operator~() const { return mint(1) / *this; }
  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 {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
  bool operator<(const mint &a) const { return x < a.x; }
  bool operator==(const mint &a) const { return x == a.x; }
};
istream &operator>>(istream &i, mint &a) {
  i >> a.x;
  return i;
}
ostream &operator<<(ostream &o, const mint &a) {
  o << a.x;
  return o;
}

int main() {
  /*
  1*2 + 1*3 + 1*4
      + 2*3 + 2*4
            + 3*4
  */
  int n;
  cin >> n;
  vector<mint> a(n);
  rep(i, n) cin >> a[i];

  mint ans = 0;
  rep(j, n) rep(i, j) ans += a[i] * a[j];
  cout << ans << endl;
  return 0;
}