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

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#include <algorithm>
#include <cmath>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <string_view>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define all(i) i.begin(), i.end()
template <typename T = ll> using v = vector<T>;
template <typename T = ll> using vv = v<v<T>>;
template <typename T = ll, typename U = ll> using p = pair<T, U>;
template <typename T> T chmin(T &a, T b) { return a = min(a, b); }
template <typename T> T chmax(T &a, T b) { return a = max(a, b); }
const ll INF = (ll)1e9;
template <int MOD> struct ModInt {
  using lint = long long;
  int val;

  // constructor
  ModInt(lint v = 0) : val(v % MOD) {
    if (val < 0)
      val += MOD;
  };

  // unary operator
  ModInt operator+() const { return ModInt(val); }
  ModInt operator-() const { return ModInt(MOD - val); }
  ModInt inv() const { return this->pow(MOD - 2); }

  // arithmetic
  ModInt operator+(const ModInt &x) const { return ModInt(*this) += x; }
  ModInt operator-(const ModInt &x) const { return ModInt(*this) -= x; }
  ModInt operator*(const ModInt &x) const { return ModInt(*this) *= x; }
  ModInt operator/(const ModInt &x) const { return ModInt(*this) /= x; }
  ModInt pow(lint n) const {
    auto x = ModInt(1);
    auto b = *this;
    while (n > 0) {
      if (n & 1)
        x *= b;
      n >>= 1;
      b *= b;
    }
    return x;
  }

  // compound assignment
  ModInt &operator+=(const ModInt &x) {
    if ((val += x.val) >= MOD)
      val -= MOD;
    return *this;
  }
  ModInt &operator-=(const ModInt &x) {
    if ((val -= x.val) < 0)
      val += MOD;
    return *this;
  }
  ModInt &operator*=(const ModInt &x) {
    val = lint(val) * x.val % MOD;
    return *this;
  }
  ModInt &operator/=(const ModInt &x) { return *this *= x.inv(); }

  // compare
  bool operator==(const ModInt &b) const { return val == b.val; }
  bool operator!=(const ModInt &b) const { return val != b.val; }

  // I/O
  friend std::istream &operator>>(std::istream &is, ModInt &x) noexcept {
    return is >> x.val;
  }
  friend std::ostream &operator<<(std::ostream &os, const ModInt &x) noexcept {
    return os << x.val;
  }
};

constexpr int MOD = 1e9 + 7;
using mint = ModInt<MOD>;
void a() {
  ll D, T, S;
  cin >> D >> T >> S;
  if (D <= T * S) {
    cout << "Yes" << endl;
  } else {
    cout << "No" << endl;
  }
}

void b() {
  string S, T;
  cin >> S >> T;
  ll ans = 0;
  for (ll i = 0; i < S.size() - T.size(); i++) {
    ll cnt = 0;
    for (ll j = 0; j < T.size(); j++) {
      if (S[i + j] == T[j])
        cnt++;
    }
    chmax(ans, cnt);
  }
  cout << T.size() - ans << endl;
}

void c() {
  ll N;
  cin >> N;
  auto A = v<mint>(N, 0);
  rep(i, N) cin >> A[i];
  mint ans = 0;
  mint sum = 0;
  rep(i, N) sum += A[i];

  for (ll i = N - 1; i >= 0; i--) {
    ans += (sum - A[i]) * A[i];
  }
  cout << ans / 2 << endl;
}

void d() {}

void e() {}

int main(void) {
  c();
  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#include <algorithm>
#include <cmath>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <string_view>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define all(i) i.begin(), i.end()
template <typename T = ll> using v = vector<T>;
template <typename T = ll> using vv = v<v<T>>;
template <typename T = ll, typename U = ll> using p = pair<T, U>;
template <typename T> T chmin(T &a, T b) { return a = min(a, b); }
template <typename T> T chmax(T &a, T b) { return a = max(a, b); }
const ll INF = (ll)1e9;
template <int MOD> struct ModInt {
  using lint = long long;
  int val;

  // constructor
  ModInt(lint v = 0) : val(v % MOD) {
    if (val < 0)
      val += MOD;
  };

  // unary operator
  ModInt operator+() const { return ModInt(val); }
  ModInt operator-() const { return ModInt(MOD - val); }
  ModInt inv() const { return this->pow(MOD - 2); }

  // arithmetic
  ModInt operator+(const ModInt &x) const { return ModInt(*this) += x; }
  ModInt operator-(const ModInt &x) const { return ModInt(*this) -= x; }
  ModInt operator*(const ModInt &x) const { return ModInt(*this) *= x; }
  ModInt operator/(const ModInt &x) const { return ModInt(*this) /= x; }
  ModInt pow(lint n) const {
    auto x = ModInt(1);
    auto b = *this;
    while (n > 0) {
      if (n & 1)
        x *= b;
      n >>= 1;
      b *= b;
    }
    return x;
  }

  // compound assignment
  ModInt &operator+=(const ModInt &x) {
    if ((val += x.val) >= MOD)
      val -= MOD;
    return *this;
  }
  ModInt &operator-=(const ModInt &x) {
    if ((val -= x.val) < 0)
      val += MOD;
    return *this;
  }
  ModInt &operator*=(const ModInt &x) {
    val = lint(val) * x.val % MOD;
    return *this;
  }
  ModInt &operator/=(const ModInt &x) { return *this *= x.inv(); }

  // compare
  bool operator==(const ModInt &b) const { return val == b.val; }
  bool operator!=(const ModInt &b) const { return val != b.val; }

  // I/O
  friend std::istream &operator>>(std::istream &is, ModInt &x) noexcept {
    return is >> x.val;
  }
  friend std::ostream &operator<<(std::ostream &os, const ModInt &x) noexcept {
    return os << x.val;
  }
};

constexpr int MOD = 1e9 + 7;
using mint = ModInt<MOD>;
void a() {
  ll D, T, S;
  cin >> D >> T >> S;
  if (D <= T * S) {
    cout << "Yes" << endl;
  } else {
    cout << "No" << endl;
  }
}

void b() {
  string S, T;
  cin >> S >> T;
  ll ans = 0;
  for (ll i = 0; i < S.size() - T.size(); i++) {
    ll cnt = 0;
    for (ll j = 0; j < T.size(); j++) {
      if (S[i + j] == T[j])
        cnt++;
    }
    chmax(ans, cnt);
  }
  cout << T.size() - ans << endl;
}

void c() {
  ll N;
  cin >> N;
  auto A = v<mint>(N, 0);
  rep(i, N) cin >> A[i];
  mint ans;
  rep(i, N) {
    for (ll j = i + 1; j < N; j++) {
      ans += A[i] * A[j];
    }
  }
  cout << ans << endl;
}

void d() {}

void e() {}

int main(void) {
  c();
  return 0;
}