Language: C++

#include <bits/stdc++.h>
#define int int64_t
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++ (i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++ (i))
#define REP_R(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define REP3R(i, m, n) for (int i = (int)(n) - 1; (i) >= (int)(m); -- (i))
#define REPP(i, n) for (int i = 1; (i) < (int)(n+1); ++ (i))
#define REPR(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define FOR(i, m, n) for(int i = m; i < n; i++)
#define FORR(i, m, n) for(int i=(m)-1; i>=(n); i--)
#define ALL(v) v.begin(), v.end()
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(),(c).end())
#define SORTR(c) sort((c).begin(),(c).end(),greater<int>());
#define BIT(n) (1LL<<(n))
#define PCNT(x) __builtin_popcountll(x)
#define GCD(x,y) __gcd(x,y)
#define LCM(x,y) x/__gcd(x,y)*y
#define PB push_back
#define MP make_pair
#define endl "\n"
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
using namespace std;
using Graph = vector<vector<int> >;
using WeightedGraph = vector<vector<pair<int, int> > >;
typedef long double LD;
typedef vector<int> VI;
typedef vector<vector<int> > VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
const int INF = 2000000000;
const long double PI = acosl(-1);
const int MOD = 1000000007;
void print() {cout << endl;}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head; if (sizeof...(tail) != 0) cout << " ";print(forward<Tail>(tail)...);}
template <class T> void print(vector<T> &vec) {for (auto& a : vec) {cout << a; if (&a != &vec.back()) cout << " ";} cout << endl;}
template <class T> void print(vector<vector<T>> &df) { for (auto& vec : df) { print(vec);}}

struct mint {
    int x;
    mint(int x=0):x((x%MOD+MOD)%MOD){}
    mint operator-() const { return mint(-x);}
    mint& operator+=(const mint a) {
        if ((x += a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator-=(const mint a) {
        if ((x += MOD-a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator*=(const mint a) { (x *= a.x) %= MOD; return *this;}
    mint operator+(const mint a) const { return mint(*this) += a;}
    mint operator-(const mint a) const { return mint(*this) -= a;}
    mint operator*(const mint a) const { return mint(*this) *= a;}
    mint pow(int t) const {
        if (!t) return 1;
        mint a = pow(t>>1);
        a *= a;
        if (t&1) a *= *this;
        return a;
    }
    mint inv() const { return pow(MOD-2);}
    mint& operator/=(const mint a) { return *this *= a.inv();}
    mint operator/(const mint a) const { return mint(*this) /= a;}
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}


signed main() {
  	chrono::system_clock::time_point start_time, end_time;
    start_time = chrono::system_clock::now();
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    cout << fixed << setprecision(10);
    mint ans = 0;
    
    int N;
    cin >> N;
    vector<int> A(N);
    VI B(N+1,0);
    REP (i, N) {
        cin >> A[i];
        B[i+1] = B[i] + A[i];
        B[i+1] %= MOD;
    }
    REP(i,N) {
        mint tmp = B[N] - B[i+1];
        tmp *= A[i];
        ans += tmp;
    }
    cout << ans << endl;

    
    
    end_time = chrono::system_clock::now();
    assert(chrono::duration_cast<chrono::milliseconds>(end_time-start_time).count() < 30)
    return 0;
}

/*
cd ~/Desktop/Programming/AtCoder/contest/abc177/abc177_c && sh shellscripts/test_cpp.sh
cd ~/Desktop/Programming/AtCoder/contest/abc177/abc177_c && sh shellscripts/etest_cpp.sh
cd ~/Desktop/Programming/AtCoder/contest/abc177/abc177_c && sh shellscripts/submit_cpp.sh
*/


Compiler output:
a.cc: In function 'int main()':
a.cc:100:5: error: expected ';' before 'return'
  100 |     return 0;
      |     ^~~~~~