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

///   BISMILLAHIR RAHMANIR RAHEEM
///   ALLAH IS WATCHING ME
///   ALLAH save us from COVID-19.Amin.

///   Every soul shall taste death.

///   Tag ::

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/detail/standard_policies.hpp>
#include <ext/pb_ds/tree_policy.hpp>

///  order_of_key return number of elements less than x.
///  find_by_order return index.

using namespace std;
using namespace __gnu_pbds;

#define MOHAMMAD                                                               \\
  ios::sync_with_stdio(0);                                                     \\
  cin.tie(0);
#define all(x) (x).begin(), (x).end()
#define AE cout << fixed << setprecision(2);

/// faster.
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")

int dx8[] = {0, 0, 1, 1, 1, -1, -1, -1}; // 8-direction.......
int dy8[] = {1, -1, 1, -1, 0, 0, -1, 1};
int dx4[] = {1, -1, 0, 0}; // 4-direction...........
int dy4[] = {0, 0, 1, -1};

using ll = long long;
const ll MOD = 1e9 + 7;

typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    ordered_set;
typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag,
             tree_order_statistics_node_update>
    ordered_multiset;

// debug
template <typename F, typename S>
ostream &operator<<(ostream &os, const pair<F, S> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "{";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  return os << "}";
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ",";
    os << *it;
  }
  return os << "]";
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  return os << "]";
}
template <typename F, typename S>
ostream &operator<<(ostream &os, const map<F, S> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << it->first << " = " << it->second;
  }
  return os << "]";
}
#define dbg(args...)                                                           \\
  do {                                                                         \\
    cerr << #args << " : ";                                                    \\
    faltu(args);                                                               \\
  } while (0)
void faltu() { cerr << endl; }
template <typename T> void faltu(T a[], int n) {
  for (int i = 0; i < n; ++i)
    cerr << a[i] << ' ';
  cerr << endl;
}
template <typename T, typename... hello>
void faltu(T arg, const hello &...rest) {
  cerr << arg << ' ';
  faltu(rest...);
}
// #else
// #define dbg(args...)

/// Modular arithmetic
inline void normal(ll &a) {
  a %= MOD;
  (a < 0) && (a += MOD);
}
inline ll modMul(ll a, ll b) {
  a %= MOD, b %= MOD;
  normal(a), normal(b);
  return (a * b) % MOD;
}
inline ll modAdd(ll a, ll b) {
  a %= MOD, b %= MOD;
  normal(a), normal(b);
  return (a + b) % MOD;
}
inline ll modSub(ll a, ll b) {
  a %= MOD, b %= MOD;
  normal(a), normal(b);
  a -= b;
  normal(a);
  return a;
}
inline ll modPow(ll b, ll p) {
  ll r = 1;
  while (p) {
    if (p & 1)
      r = modMul(r, b);
    b = modMul(b, b);
    p >>= 1;
  }
  return r;
}
inline ll modInverse(ll a) { return modPow(a, MOD - 2); } /// When MOD is prime.
inline ll modDiv(ll a, ll b) { return modMul(a, modInverse(b)); }

//***********************************************  The END
//**********************************************************************************************************************************
const ll N = 1e3;
const ll INF = 1e15;
const ll limit = (1LL << 64) - 1;

int main() {

  MOHAMMAD

  ll n, ans = 0;
  cin >> n;
  ll arr[n], sum = 0;
  for (ll i = 0; i < n; ++i) {
    cin >> arr[i];
    sum += arr[i];
    sum %= MOD;
  }

  for (ll i = 0; i < n; ++i) {
    ll x = arr[i];
    sum = modSub(sum, x);
    x = modMul(x, sum);
    ans = modAdd(ans, x);
  }

  cout << ans << "\\n";
}

/*

Explanation:



TODO :

  Alhamdulillah
*/


Error: TLE

Fixed submission:

///   BISMILLAHIR RAHMANIR RAHEEM
///   ALLAH IS WATCHING ME
///   ALLAH save us from COVID-19.Amin.

///   Every soul shall taste death.

///   Tag ::

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/detail/standard_policies.hpp>
#include <ext/pb_ds/tree_policy.hpp>

///  order_of_key return number of elements less than x.
///  find_by_order return index.

using namespace std;
using namespace __gnu_pbds;

#define MOHAMMAD                                                               \\
  ios::sync_with_stdio(0);                                                     \\
  cin.tie(0);
#define all(x) (x).begin(), (x).end()
#define AE cout << fixed << setprecision(2);

/// faster.
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")

int dx8[] = {0, 0, 1, 1, 1, -1, -1, -1}; // 8-direction.......
int dy8[] = {1, -1, 1, -1, 0, 0, -1, 1};
int dx4[] = {1, -1, 0, 0}; // 4-direction...........
int dy4[] = {0, 0, 1, -1};

using ll = long long;
const ll MOD = 1e9 + 7;

typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    ordered_set;
typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag,
             tree_order_statistics_node_update>
    ordered_multiset;

// debug
template <typename F, typename S>
ostream &operator<<(ostream &os, const pair<F, S> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "{";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  return os << "}";
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ",";
    os << *it;
  }
  return os << "]";
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  return os << "]";
}
template <typename F, typename S>
ostream &operator<<(ostream &os, const map<F, S> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin())
      os << ", ";
    os << it->first << " = " << it->second;
  }
  return os << "]";
}
#define dbg(args...)                                                           \\
  do {                                                                         \\
    cerr << #args << " : ";                                                    \\
    faltu(args);                                                               \\
  } while (0)
void faltu() { cerr << endl; }
template <typename T> void faltu(T a[], int n) {
  for (int i = 0; i < n; ++i)
    cerr << a[i] << ' ';
  cerr << endl;
}
template <typename T, typename... hello>
void faltu(T arg, const hello &...rest) {
  cerr << arg << ' ';
  faltu(rest...);
}
// #else
// #define dbg(args...)

/// Modular arithmetic
inline void normal(ll &a) {
  a %= MOD;
  (a < 0) && (a += MOD);
}
inline ll modMul(ll a, ll b) {
  a %= MOD, b %= MOD;
  normal(a), normal(b);
  return (a * b) % MOD;
}
inline ll modAdd(ll a, ll b) {
  a %= MOD, b %= MOD;
  normal(a), normal(b);
  return (a + b) % MOD;
}
inline ll modSub(ll a, ll b) {
  a %= MOD, b %= MOD;
  normal(a), normal(b);
  a -= b;
  normal(a);
  return a;
}
inline ll modPow(ll b, ll p) {
  ll r = 1;
  while (p) {
    if (p & 1)
      r = modMul(r, b);
    b = modMul(b, b);
    p >>= 1;
  }
  return r;
}
inline ll modInverse(ll a) { return modPow(a, MOD - 2); } /// When MOD is prime.
inline ll modDiv(ll a, ll b) { return modMul(a, modInverse(b)); }

//***********************************************  The END
//**********************************************************************************************************************************
const ll N = 1e3;
const ll INF = 1e15;
const ll limit = (1LL << 64) - 1;

int main() {

  MOHAMMAD

  ll n, sum = 0;
  cin >> n;
  ll arr[n];
  for (ll i = 0; i < n; ++i)
    cin >> arr[i];

  for (ll i = 0; i < n; ++i) {
    for (ll j = i + 1; j < n; ++j) {
      sum += modMul(arr[i], arr[j]);
      sum %= MOD;
    }
  }

  cout << sum << "\\n";
}

/*

Explanation:



TODO :

  Alhamdulillah
*/