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

#include <bits/stdc++.h>

/*
 */

#define ff first
#define ss second
#define pb push_back
#define pf push_front
#define ins insert
#define mp make_pair
#define len(s) s.length()
#define forp(i, a, b) for (int i = a; i <= b; i++)
#define rep(i, n) for (int i = 0; i < n; i++)
#define ren(i, n) for (int i = n - 1; i >= 0; i--)
#define forn(i, a, b) for (int i = a; i >= b; i--)
#define on cout << endl
#define o2(a, b) cout << a << " " << b
#define os cout << " "
#define all(v) v.begin(), v.end()
#define mem(n, m) memset(n, m, sizeof(n))
#define pii pair<int, int>
#define pll pair<long long, long long>
#define sii set<int>
#define us unordered_set<int>
#define um unordered_map<int, int>
#define sll set<long long>
#define vii vector<int>
#define vll vector<long long>
#define mll map<long long, long long>
#define lob lower_bound
#define upb upper_bound
#define ret return 0
#define present(s, x) (s.find(x) != s.end())
#define cpresent(s, x) (find(all(s), x) != s.end())
#define ford(container, it)                                                    \\
  for (__typeof(container.begin()) it = container.begin();                     \\
       it != container.end(); it++)
#define fors(container, it, a, b)                                              \\
  for (__typeof(container.begin()) it = a; it != b; it++)

#define boost ios_base::sync_with_stdio(0)
#define MOD 1000000007
#define EPSILON 1e-9
#define PI 3.14159265358979323846
#define inf 999999999999999999
#define siz 100001
#define SIZ 1000001
#define SIZE 200001

typedef long long ll;
typedef long double ldo;
typedef double db;
using namespace std;
#define pqb priority_queue<int>
#define pqs priority_queue<int, vi, greater<int>>
#define setbits(x) __builtin_popcountll(x)
#define zrobits(x) __builtin_ctzll(x)
#define w(x)                                                                   \\
  int x;                                                                       \\
  cin >> x;                                                                    \\
  while (x--)

// Everything is temporary...So,enjoy little things...
void c_p_c() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
}
int main() {
  c_p_c();
  ll n;
  cin >> n;
  ll a;
  ll sum = 0, ans = 0;
  for (ll i = 0; i < n; i++) {
    cin >> a;
    if (i) {
      ans = (ans + ((sum * a) % MOD)) % MOD;
    }
    sum = (sum + a) % MOD;
  }
  cout << ans << endl;
  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>

/*
 */

#define ff first
#define ss second
#define pb push_back
#define pf push_front
#define ins insert
#define mp make_pair
#define len(s) s.length()
#define forp(i, a, b) for (int i = a; i <= b; i++)
#define rep(i, n) for (int i = 0; i < n; i++)
#define ren(i, n) for (int i = n - 1; i >= 0; i--)
#define forn(i, a, b) for (int i = a; i >= b; i--)
#define on cout << endl
#define o2(a, b) cout << a << " " << b
#define os cout << " "
#define all(v) v.begin(), v.end()
#define mem(n, m) memset(n, m, sizeof(n))
#define pii pair<int, int>
#define pll pair<long long, long long>
#define sii set<int>
#define us unordered_set<int>
#define um unordered_map<int, int>
#define sll set<long long>
#define vii vector<int>
#define vll vector<long long>
#define mll map<long long, long long>
#define lob lower_bound
#define upb upper_bound
#define ret return 0
#define present(s, x) (s.find(x) != s.end())
#define cpresent(s, x) (find(all(s), x) != s.end())
#define ford(container, it)                                                    \\
  for (__typeof(container.begin()) it = container.begin();                     \\
       it != container.end(); it++)
#define fors(container, it, a, b)                                              \\
  for (__typeof(container.begin()) it = a; it != b; it++)

#define boost ios_base::sync_with_stdio(0)
#define MOD 1000000007
#define EPSILON 1e-9
#define PI 3.14159265358979323846
#define inf 999999999999999999
#define siz 100001
#define SIZ 1000001
#define SIZE 200001

typedef long long ll;
typedef long double ldo;
typedef double db;
using namespace std;
#define pqb priority_queue<int>
#define pqs priority_queue<int, vi, greater<int>>
#define setbits(x) __builtin_popcountll(x)
#define zrobits(x) __builtin_ctzll(x)
#define w(x)                                                                   \\
  int x;                                                                       \\
  cin >> x;                                                                    \\
  while (x--)

// Everything is temporary...So,enjoy little things...
void c_p_c() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
}
int main() {
  // c_p_c();

  ll n;
  cin >> n;
  vll v(n);
  for (ll i = 0; i < n; i++) {
    cin >> v[i];
  }
  ll sum = 0;
  ll mod = 1e9 + 7;
  for (ll i = 0; i < n - 1; i++) {
    for (ll j = i + 1; j < n; j++) {
      sum = (sum % mod + ((v[i] % mod) * (v[j] % mod))) % mod;
    }
  }
  cout << sum << endl;
  return 0;
}