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

/*
  Task: I. Kevin and Grid
  Link: http://codeforces.com/contest/1392/problem/I
  Lang: cpp17
*/
#include <bits/stdc++.h>
using namespace std;

#define eb emplace_back
#define pb push_back
#define INF LLONG_MAX
#define FLT_INF pow(10.0, 16)

using ll = long long;
using vll = vector<ll>;
using ull = unsigned long long;
using int2 = pair<ll, ll>;
using int3 = tuple<ll, ll, ll>;

template <typename A> string to_string(const A &v);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }

template <typename A, typename B> string to_string(const pair<A, B> &p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(const tuple<A, B, C> &p) {
  return "(" + to_string(std::get<0>(p)) + ", " + to_string(std::get<1>(p)) +
         ", " + to_string(std::get<2>(p)) + ")";
}

template <typename A> string to_string(const A &v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename T> void sort(vector<T> &val) {
  sort(val.begin(), val.end());
}

template <typename T> void reverse(vector<T> &val) {
  reverse(val.begin(), val.end());
}

template <typename T> void sort_b(vector<T> &val) {
  sort(val.begin(), val.end(), greater<T>());
}

template <typename T> T pair_sum(pair<T, T> &val) {
  return val.first + val.second;
}

template <typename T> T min(T a, T b, T c) { return min(min(a, b), c); }

template <typename T> T max(T a, T b, T c) { return max(max(a, b), c); }

template <typename T> T mul_mod(T a, T b, T mod) {
  T res = 0;
  a %= mod;
  while (b) {
    if (b & 1) {
      res = (res + a) % mod;
    }
    a = (2 * a) % mod;
    b >>= 1;
  }
  return res;
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) ;
#endif

#define COMMON_MOD pow(10, 9) + 7
#define PI 3.14159265

// sort(a.begin(), a.end(), less<ll>()); greater<ll>()
// cout<<fixed<<setprecision(20)<<(s/kl)*2;
/* printf("%lld %lld \\n", cur_n, m / 2 + 1);
sort(a.begin(), a.end(), [ ](ll& lhs, ll& rhs) {return abs(lhs) < abs(rhs);});
*/

class mytask {
public:
  mytask() {}

  int solve() {

    ll n;
    cin >> n;
    vll a(n);
    for (ll i = 0; i < n; i++) {
      cin >> a[i];
    }

    ll res = 0;
    ll mod7 = 1e9 + 7;
    vll sums(n);
    // sums[n-1] = a[n-1];
    for (ll i = n - 2; i >= 0; i--) {
      sums[i] = sums[i + 1] + a[i + 1];
    }
    debug(sums);
    for (ll i = 0; i < n; i++) {

      res = res + mul_mod(a[i], sums[i], mod7);
      res = res % mod7;
    }

    cout << res;

    return 0;
  }
};

int main() {

  ios::sync_with_stdio(false);
  cin.tie(0);

  auto task = mytask();
  task.solve();

  // cout << endl;

  return 0;
}


Error: TLE

Fixed submission:

/*
  Task: I. Kevin and Grid
  Link: http://codeforces.com/contest/1392/problem/I
  Lang: cpp17
*/
#include <bits/stdc++.h>
using namespace std;

#define eb emplace_back
#define pb push_back
#define INF LLONG_MAX
#define FLT_INF pow(10.0, 16)

using ll = long long;
using vll = vector<ll>;
using ull = unsigned long long;
using int2 = pair<ll, ll>;
using int3 = tuple<ll, ll, ll>;

template <typename A> string to_string(const A &v);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }

template <typename A, typename B> string to_string(const pair<A, B> &p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(const tuple<A, B, C> &p) {
  return "(" + to_string(std::get<0>(p)) + ", " + to_string(std::get<1>(p)) +
         ", " + to_string(std::get<2>(p)) + ")";
}

template <typename A> string to_string(const A &v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename T> void sort(vector<T> &val) {
  sort(val.begin(), val.end());
}

template <typename T> void reverse(vector<T> &val) {
  reverse(val.begin(), val.end());
}

template <typename T> void sort_b(vector<T> &val) {
  sort(val.begin(), val.end(), greater<T>());
}

template <typename T> T pair_sum(pair<T, T> &val) {
  return val.first + val.second;
}

template <typename T> T min(T a, T b, T c) { return min(min(a, b), c); }

template <typename T> T max(T a, T b, T c) { return max(max(a, b), c); }

template <typename T> T mul_mod(T a, T b, T mod) {
  T res = 0;
  a %= mod;
  while (b) {
    if (b & 1) {
      res = (res + a) % mod;
    }
    a = (2 * a) % mod;
    b >>= 1;
  }
  return res;
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) ;
#endif

#define COMMON_MOD pow(10, 9) + 7
#define PI 3.14159265

// sort(a.begin(), a.end(), less<ll>()); greater<ll>()
// cout<<fixed<<setprecision(20)<<(s/kl)*2;
/* printf("%lld %lld \\n", cur_n, m / 2 + 1);
sort(a.begin(), a.end(), [ ](ll& lhs, ll& rhs) {return abs(lhs) < abs(rhs);});
*/

class mytask {
public:
  mytask() {}

  int solve() {

    ll n;
    cin >> n;
    vll a(n);
    for (ll i = 0; i < n; i++) {
      cin >> a[i];
    }

    ll res = 0;
    ll mod7 = 1e9 + 7;
    for (ll i = 0; i < n; i++) {
      for (ll j = i + 1; j < n; j++) {
        res = res + (a[i] * a[j]) % mod7;
        res = res % mod7;
      }
    }

    cout << res;

    return 0;
  }
};

int main() {

  ios::sync_with_stdio(false);
  cin.tie(0);

  auto task = mytask();
  task.solve();

  // cout << endl;

  return 0;
}