Problem p02572 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
using namespace std;
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// using namespace __gnu_pbds;
#define fastio()                                                               \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0)
#define pb push_back
#define show(x) cout << (#x) << " : " << x << endl;
#define ll long long
#define ld long double
#define fill(a, val) memset(a, val, sizeof(a))
#define mp make_pair
#define ff first
#define ss second
#define pii pair<ll, ll>
#define sq(x) ((x) * (x))
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define endl "\n"
#define int long long
#define printclock                                                             \
  cerr << "Time : " << 1000 * (ld)clock() / (ld)CLOCKS_PER_SEC << "ms\n";
// #define ordered_set tree<int, null_type,less<int>,
// rb_tree_tag,tree_order_statistics_node_update>
const ll mod = 1000 * 1000 * 1000 + 7;
const ll INF = 1ll * 1000 * 1000 * 1000 * 1000 * 1000 * 1000 + 7;
const ll MOD2 = 998244353;
const ll N = 1000 * 1000 + 10;
const ll N2 = 70;
const ld PI = 3.141592653589793;
// template<class T>using onset=tree <T, null_type, less<T>, rb_tree_tag,
// tree_order_statistics_node_update>;
ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return gcd(b, a % b);
}
ll power(ll x, ll y, ll p = LLONG_MAX) {
  ll res = 1;
  x %= p;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
void solve() {
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
}
signed main() {
  fastio();
  // cout<<fixed<<setprecision(20);
  ll n;
  cin >> n;
  ll a[n];
  for (ll i = 0; i < n; i++) {
    cin >> a[i];
  }
  ll ans = 0;
  for (ll i = 1; i < n; i++) {
    for (ll j = 0; j < n; j++) {
      if ((j + i) >= n) {
        break;
      }
      // cout<<j<<" "<<(j+i)<<endl;
      ans += (a[j] % mod * a[(j + i)] % mod) % mod;
      ans %= mod;
    }
  }
  cout << ans;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
using namespace std;
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// using namespace __gnu_pbds;
#define fastio()                                                               \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0)
#define pb push_back
#define show(x) cout << (#x) << " : " << x << endl;
#define ll long long
#define ld long double
#define fill(a, val) memset(a, val, sizeof(a))
#define mp make_pair
#define ff first
#define ss second
#define pii pair<ll, ll>
#define sq(x) ((x) * (x))
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define endl "\n"
#define int long long
#define printclock                                                             \
  cerr << "Time : " << 1000 * (ld)clock() / (ld)CLOCKS_PER_SEC << "ms\n";
// #define ordered_set tree<int, null_type,less<int>,
// rb_tree_tag,tree_order_statistics_node_update>
const ll mod = 1000 * 1000 * 1000 + 7;
const ll INF = 1ll * 1000 * 1000 * 1000 * 1000 * 1000 * 1000 + 7;
const ll MOD2 = 998244353;
const ll N = 1000 * 1000 + 10;
const ll N2 = 70;
const ld PI = 3.141592653589793;
// template<class T>using onset=tree <T, null_type, less<T>, rb_tree_tag,
// tree_order_statistics_node_update>;
ll gcd(ll a, ll b) {
  if (!b)
    return a;
  return gcd(b, a % b);
}
ll power(ll x, ll y, ll p = LLONG_MAX) {
  ll res = 1;
  x %= p;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
void solve() {
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
}
signed main() {
  fastio();
  // cout<<fixed<<setprecision(20);
  ll n;
  cin >> n;
  ll a[n];
  for (ll i = 0; i < n; i++) {
    cin >> a[i];
  }
  ll ans = 0, sum = 0;
  for (ll i = 0; i < n; i++) {
    sum += a[i];
  }
  for (ll i = 0; i < n; i++) {
    sum -= a[i];
    ans = ans % mod + (sum % mod * a[i] % mod) % mod;
  }
  cout << ans;
  return 0;
}