Problem p02572 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;

// long long
using ll = long long;

// pair<int, int>
using PII = pair<int, int>;

// 最大値、mod
const int MOD = 1000000007;
const int mod = 1000000007;
const int INF = 1000000000;
const long long LINF = 1e18;
const int MAX = 510000;

// 出力系
#define print(x) cout << x << endl
#define prints(x) cout << fixed << setprecision(20) << x << endl
#define printc(x) cout << setw(2) << setfill('0') << x << endl;
#define yes cout << "Yes" << endl
#define YES cout << "YES" << endl
#define no cout << "No" << endl
#define NO cout << "NO" << endl

// 配列入力
vector<long long> vecin(ll n) {
  vector<long long> res(n);
  for (int i = 0; i < n; i++)
    cin >> res[i];
  return res;
}

// begin() end()
#define all(x) (x).begin(), (x).end()

// for
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define rrep(i, a, b) for (int i = (a); i > (b); i--)
#define rep(i, a, b) for (int i = (a); i < (b); i++)

// 最大公約数
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }

// 最小公倍数
unsigned lcm(unsigned a, unsigned b) { return a / gcd(a, b) * b; }

// a = max(a, b), a = min(a, b)
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

// num ^ pow(mod取る)
ll pow_mod(ll num, ll pow, ll mod) {
  ll prod = 1;
  num %= mod;
  while (pow > 0) {
    if (pow & 1)
      prod = prod * num % mod;
    num = num * num % mod;
    pow >>= 1;
  }
  return prod;
}

// 二項係数(MODとる、1 ≦ k ≦ n ≦ 10^7 程度)
// COMinit()
// COM(x, y)
// とコンビで使う

// テーブルを作る前処理
long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

// 重みつきUnionFInd
template <class Abel> struct GUnionFind {
  vector<int> par;
  vector<int> rank;
  vector<Abel> diff_weight;

  GUnionFind(int n = 1, Abel SUM_UNITY = 0) { init(n, SUM_UNITY); }

  void init(int n = 1, Abel SUM_UNITY = 0) {
    par.resize(n);
    rank.resize(n);
    diff_weight.resize(n);
    for (int i = 0; i < n; ++i)
      par[i] = i, rank[i] = 0, diff_weight[i] = SUM_UNITY;
  }

  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      diff_weight[x] += diff_weight[par[x]];
      return par[x] = r;
    }
  }

  Abel weight(int x) {
    root(x);
    return diff_weight[x];
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  bool merge(int x, int y, Abel w) {
    w += weight(x);
    w -= weight(y);
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y), w = -w;
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    diff_weight[y] = w;
    return true;
  }

  Abel diff(int x, int y) { return weight(y) - weight(x); }
};

// UnionFind
struct UnionFind {
  vector<int> par;
  vector<int> rank;
  vector<ll> Size;
  UnionFind(int n = 1) { init(n); }

  void init(int n = 1) {
    par.resize(n + 1);
    rank.resize(n + 1);
    Size.resize(n + 1);
    for (int i = 0; i <= n; ++i)
      par[i] = i, rank[i] = 0, Size[i] = 1;
  }

  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    Size[x] += Size[y];
    return true;
  }

  ll size(int x) { return Size[root(x)]; }
};

// modint構造体
struct Mint {
  int val;
  Mint inv() const {
    int tmp, a = val, b = mod, x = 1, y = 0;
    while (b)
      tmp = a / b, a -= tmp * b, swap(a, b), x -= tmp * y, swap(x, y);
    return Mint(x);
  }

public:
  Mint() : val(0) {}
  Mint(ll x) {
    if ((val = x % mod) < 0)
      val += mod;
  }
  Mint pow(ll t) {
    Mint res = 1, b = *this;
    while (t) {
      if (t & 1)
        res *= b;
      b *= b;
      t >>= 1;
    }
    return res;
  }
  Mint &operator+=(const Mint &x) {
    if ((val += x.val) >= mod)
      val -= mod;
    return *this;
  }
  Mint &operator-=(const Mint &x) {
    if ((val += mod - x.val) >= mod)
      val -= mod;
    return *this;
  }
  Mint &operator*=(const Mint &x) {
    val = (ll)val * x.val % mod;
    return *this;
  }
  Mint &operator/=(const Mint &x) { return *this *= x.inv(); }
  bool operator==(const Mint &x) const { return val == x.val; }
  bool operator!=(const Mint &x) const { return val != x.val; }
  bool operator<(const Mint &x) const { return val < x.val; }
  bool operator<=(const Mint &x) const { return val <= x.val; }
  bool operator>(const Mint &x) const { return val > x.val; }
  bool operator>=(const Mint &x) const { return val >= x.val; }
  Mint operator+(const Mint &x) const { return Mint(*this) += x; }
  Mint operator-(const Mint &x) const { return Mint(*this) -= x; }
  Mint operator*(const Mint &x) const { return Mint(*this) *= x; }
  Mint operator/(const Mint &x) const { return Mint(*this) /= x; }
};
struct factorial {
  vector<Mint> Fact, Finv;

public:
  // factorial fact(10000010);
  // fact.nCr(a, b)
  // 「fact」の部分は自由に名前変更可能
  factorial(int maxx) {
    Fact.resize(maxx + 1), Finv.resize(maxx + 1);
    Fact[0] = Mint(1);
    rep(i, 0, maxx) Fact[i + 1] = Fact[i] * (i + 1);
    Finv[maxx] = Mint(1) / Fact[maxx];
    rrep(i, maxx, 0) Finv[i - 1] = Finv[i] * i;
  }
  Mint fact(int n, bool inv = 0) {
    if (inv)
      return Finv[n];
    else
      return Fact[n];
  }
  Mint nPr(int n, int r) {
    if (n < 0 || n < r || r < 0)
      return Mint(0);
    else
      return Fact[n] * Finv[n - r];
  }
  Mint nCr(int n, int r) {
    if (n < 0 || n < r || r < 0)
      return Mint(0);
    else
      return Fact[n] * Finv[r] * Finv[n - r];
  }
};

// 1 * 2 * 3 .... * n (mod)
ll modfact(ll n) {
  if (n <= 1)
    return 1;
  return (n * modfact(n - 1)) % MOD;
}

// kが角度だった場合:cos(k * (PI / 180));
// const double PI = acos(-1);のまま使うと円周率(M_PIもあるよ)
const double PI = acos(-1);

// 多次元 vector 生成   例: auto dp = make_vec<long long>(N+1, 5, 5, 5);
template <class T> vector<T> make_vec(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto make_vec(size_t a, Ts... ts) {
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

// 素因数分解
vector<pair<long long, int>> factorize(long long n) {
  vector<pair<long long, int>> res;
  for (long long i = 2; i * i <= n; ++i) {
    if (n % i)
      continue;
    res.emplace_back(i, 0);
    while (n % i == 0) {
      n /= i;
      res.back().second++;
    }
  }
  if (n != 1)
    res.emplace_back(n, 1);
  return res;
}

// 素数判定
bool primejudge(long long a) {
  if (a <= 1)
    return false;
  for (long long i = 2; i * i <= a; i++) {
    if (a % i == 0)
      return false;
  }
  return true;
}

// 　累積和
//  vector<long long>sums(vector<int>n){
//      vector<long long>res(n.size() + 1, 0);
//      for(int i = 0; i < n.size(); i++) res[i + 1] = n[i] + res[i];
//      return res;
//  }

int dy[4] = {0, 1, 0, -1}, dx[4] = {1, 0, -1, 0};
vector<vector<int>> graph;

int main() {
  int n;
  cin >> n;
  vector<ll> a(n);
  REP(i, n) cin >> a[i];
  Mint ans = 0;
  REP(i, n) {
    REP(j, i) { ans += a[i] * a[j]; }
  }
  print(ans.val);
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

// long long
using ll = long long;

// pair<int, int>
using PII = pair<int, int>;

// 最大値、mod
const int MOD = 1000000007;
const int mod = 1000000007;
const int INF = 1000000000;
const long long LINF = 1e18;
const int MAX = 510000;

// 出力系
#define print(x) cout << x << endl
#define prints(x) cout << fixed << setprecision(20) << x << endl
#define printc(x) cout << setw(2) << setfill('0') << x << endl;
#define yes cout << "Yes" << endl
#define YES cout << "YES" << endl
#define no cout << "No" << endl
#define NO cout << "NO" << endl

// 配列入力
vector<long long> vecin(ll n) {
  vector<long long> res(n);
  for (int i = 0; i < n; i++)
    cin >> res[i];
  return res;
}

// begin() end()
#define all(x) (x).begin(), (x).end()

// for
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define rrep(i, a, b) for (int i = (a); i > (b); i--)
#define rep(i, a, b) for (int i = (a); i < (b); i++)

// 最大公約数
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }

// 最小公倍数
unsigned lcm(unsigned a, unsigned b) { return a / gcd(a, b) * b; }

// a = max(a, b), a = min(a, b)
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

// num ^ pow(mod取る)
ll pow_mod(ll num, ll pow, ll mod) {
  ll prod = 1;
  num %= mod;
  while (pow > 0) {
    if (pow & 1)
      prod = prod * num % mod;
    num = num * num % mod;
    pow >>= 1;
  }
  return prod;
}

// 二項係数(MODとる、1 ≦ k ≦ n ≦ 10^7 程度)
// COMinit()
// COM(x, y)
// とコンビで使う

// テーブルを作る前処理
long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

// 重みつきUnionFInd
template <class Abel> struct GUnionFind {
  vector<int> par;
  vector<int> rank;
  vector<Abel> diff_weight;

  GUnionFind(int n = 1, Abel SUM_UNITY = 0) { init(n, SUM_UNITY); }

  void init(int n = 1, Abel SUM_UNITY = 0) {
    par.resize(n);
    rank.resize(n);
    diff_weight.resize(n);
    for (int i = 0; i < n; ++i)
      par[i] = i, rank[i] = 0, diff_weight[i] = SUM_UNITY;
  }

  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      diff_weight[x] += diff_weight[par[x]];
      return par[x] = r;
    }
  }

  Abel weight(int x) {
    root(x);
    return diff_weight[x];
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  bool merge(int x, int y, Abel w) {
    w += weight(x);
    w -= weight(y);
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y), w = -w;
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    diff_weight[y] = w;
    return true;
  }

  Abel diff(int x, int y) { return weight(y) - weight(x); }
};

// UnionFind
struct UnionFind {
  vector<int> par;
  vector<int> rank;
  vector<ll> Size;
  UnionFind(int n = 1) { init(n); }

  void init(int n = 1) {
    par.resize(n + 1);
    rank.resize(n + 1);
    Size.resize(n + 1);
    for (int i = 0; i <= n; ++i)
      par[i] = i, rank[i] = 0, Size[i] = 1;
  }

  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    Size[x] += Size[y];
    return true;
  }

  ll size(int x) { return Size[root(x)]; }
};

// modint構造体
struct Mint {
  int val;
  Mint inv() const {
    int tmp, a = val, b = mod, x = 1, y = 0;
    while (b)
      tmp = a / b, a -= tmp * b, swap(a, b), x -= tmp * y, swap(x, y);
    return Mint(x);
  }

public:
  Mint() : val(0) {}
  Mint(ll x) {
    if ((val = x % mod) < 0)
      val += mod;
  }
  Mint pow(ll t) {
    Mint res = 1, b = *this;
    while (t) {
      if (t & 1)
        res *= b;
      b *= b;
      t >>= 1;
    }
    return res;
  }
  Mint &operator+=(const Mint &x) {
    if ((val += x.val) >= mod)
      val -= mod;
    return *this;
  }
  Mint &operator-=(const Mint &x) {
    if ((val += mod - x.val) >= mod)
      val -= mod;
    return *this;
  }
  Mint &operator*=(const Mint &x) {
    val = (ll)val * x.val % mod;
    return *this;
  }
  Mint &operator/=(const Mint &x) { return *this *= x.inv(); }
  bool operator==(const Mint &x) const { return val == x.val; }
  bool operator!=(const Mint &x) const { return val != x.val; }
  bool operator<(const Mint &x) const { return val < x.val; }
  bool operator<=(const Mint &x) const { return val <= x.val; }
  bool operator>(const Mint &x) const { return val > x.val; }
  bool operator>=(const Mint &x) const { return val >= x.val; }
  Mint operator+(const Mint &x) const { return Mint(*this) += x; }
  Mint operator-(const Mint &x) const { return Mint(*this) -= x; }
  Mint operator*(const Mint &x) const { return Mint(*this) *= x; }
  Mint operator/(const Mint &x) const { return Mint(*this) /= x; }
};
struct factorial {
  vector<Mint> Fact, Finv;

public:
  // factorial fact(10000010);
  // fact.nCr(a, b)
  // 「fact」の部分は自由に名前変更可能
  factorial(int maxx) {
    Fact.resize(maxx + 1), Finv.resize(maxx + 1);
    Fact[0] = Mint(1);
    rep(i, 0, maxx) Fact[i + 1] = Fact[i] * (i + 1);
    Finv[maxx] = Mint(1) / Fact[maxx];
    rrep(i, maxx, 0) Finv[i - 1] = Finv[i] * i;
  }
  Mint fact(int n, bool inv = 0) {
    if (inv)
      return Finv[n];
    else
      return Fact[n];
  }
  Mint nPr(int n, int r) {
    if (n < 0 || n < r || r < 0)
      return Mint(0);
    else
      return Fact[n] * Finv[n - r];
  }
  Mint nCr(int n, int r) {
    if (n < 0 || n < r || r < 0)
      return Mint(0);
    else
      return Fact[n] * Finv[r] * Finv[n - r];
  }
};

// 1 * 2 * 3 .... * n (mod)
ll modfact(ll n) {
  if (n <= 1)
    return 1;
  return (n * modfact(n - 1)) % MOD;
}

// kが角度だった場合:cos(k * (PI / 180));
// const double PI = acos(-1);のまま使うと円周率(M_PIもあるよ)
const double PI = acos(-1);

// 多次元 vector 生成   例: auto dp = make_vec<long long>(N+1, 5, 5, 5);
template <class T> vector<T> make_vec(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto make_vec(size_t a, Ts... ts) {
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

// 素因数分解
vector<pair<long long, int>> factorize(long long n) {
  vector<pair<long long, int>> res;
  for (long long i = 2; i * i <= n; ++i) {
    if (n % i)
      continue;
    res.emplace_back(i, 0);
    while (n % i == 0) {
      n /= i;
      res.back().second++;
    }
  }
  if (n != 1)
    res.emplace_back(n, 1);
  return res;
}

// 素数判定
bool primejudge(long long a) {
  if (a <= 1)
    return false;
  for (long long i = 2; i * i <= a; i++) {
    if (a % i == 0)
      return false;
  }
  return true;
}

// 　累積和
//  vector<long long>sums(vector<int>n){
//      vector<long long>res(n.size() + 1, 0);
//      for(int i = 0; i < n.size(); i++) res[i + 1] = n[i] + res[i];
//      return res;
//  }

int dy[4] = {0, 1, 0, -1}, dx[4] = {1, 0, -1, 0};
vector<vector<int>> graph;

int main() {
  int n;
  cin >> n;
  ll sum = 0;
  ll ans = 0;
  for (int i = 0; i < n; i++) {
    ll x;
    cin >> x;
    ans += x * sum % MOD;
    ans %= MOD;
    sum += x;
    sum %= MOD;
  }
  cout << ans << endl;
  return 0;
}