Problem p02572 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <ratio>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define ll long long
#define ld long double
#define mp make_pair
#define pb push_back
#define in insert
#define vll vector<ll>
#define endl "\\n"
#define pll pair<ll, ll>
#define f first
#define s second
#define FOR(i, a, b) for (int i = (a), _b = (b); i <= _b; i++)
#define int ll
#define sz(x) (ll) x.size()
#define all(x) (x.begin(), x.end())
using namespace std;

const ll INF = 1e12;
const ll N = (2e5 + 5); // TODO : change value as per problem
const ll MOD = 1e9 + 7;

int a[N];
void solve() {
  int ans = 0;
  int n;
  cin >> n;
  int sum = 0;
  for (int i = 1; i <= n; i++)
    cin >> a[i], sum = (sum + a[i]) % MOD;
  for (int i = 1; i <= n; i++) {
    sum -= a[i];
    if (sum < 0)
      sum += MOD;
    sum %= MOD;
    a[i] = (a[i] * sum) % MOD;
    if (a[i] < 0)
      a[i] += MOD;
    a[i] %= MOD;
    ans = (ans + a[i]) % MOD;
    if (ans < 0)
      ans += MOD;
    ans %= MOD;
  }
  cout << ans << endl;
}
signed main() {

  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  // freopen(".in","r",stdin);freopen(".out","w",stdout);

  ll tt = 1;
  // cin >> tt;
  while (tt--) {
    solve();
  }
}

Error: 0

Fixed submission:

#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <ratio>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define ll long long
#define ld long double
#define mp make_pair
#define pb push_back
#define in insert
#define vll vector<ll>
#define endl "\\n"
#define pll pair<ll, ll>
#define f first
#define s second
#define FOR(i, a, b) for (int i = (a), _b = (b); i <= _b; i++)
#define int ll
#define sz(x) (ll) x.size()
#define all(x) (x.begin(), x.end())
using namespace std;

const ll INF = 1e12;
const ll N = (1e5 + 5); // TODO : change value as per problem
const ll MOD = 1e9 + 7;

int a[N];
void solve() {
  int ans = 0;
  int n;
  cin >> n;
  int sum = 0;
  for (int i = 1; i <= n; i++)
    cin >> a[i], sum += a[i];
  for (int i = 1; i <= n; i++) {
    int cur = sum - a[i];
    cur = (cur * a[i]);
    ans = (ans + cur) % MOD;
  }
  ans /= 2;

  cout << ans << endl;
}
signed main() {

  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  // freopen(".in","r",stdin);freopen(".out","w",stdout);

  ll tt = 1;
  // cin >> tt;
  while (tt--) {
    solve();
  }
}