Problem p02572 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

#define BOUND 27182818284
#define MAT 2

typedef long long ll;
typedef long long int lli;
typedef pair<ll, ll> P;

ll MOD = 1000000007;
const ll INF = (1ll << 60);
const int INFint = (1 << 30);

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repi(i, a, b) for (int i = int(a); i < int(b); ++i)

template <class T> bool umax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

template <class T> bool umin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

// gcd
template <typename T> T gcd(T a, T b) {
  if (a == 0)
    return b;
  return gcd(b % a, a);
}

ll findGCD(vector<ll> arr) {
  ll result = arr[0];
  for (auto a : arr) {
    result = gcd(a, result);
  }
  return result;
}

template <typename T> T getlcm(T m, T n) {
  // 引数に0がある場合は0を返す
  if ((0 == m) || (0 == n))
    return 0;
  return ((m / gcd(m, n)) * n); // lcm = m * n / gcd(m,n)
}

template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  fill((T *)array, (T *)(array + N), val);
}

// v.front() = -BOUND;
// v.back() = BOUND;

// struct edge{
//     int cost, to;
//
//     edge(int in_cost, int in_to){
//         cost=in_cost;
//         to=in_to;
//     }
//     bool operator<(const edge &a) const
//     {
//         return cost > a.cost;
//     }
// };
ll euler_phi(ll n) {
  ll ret = n;
  for (ll i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      ret -= ret / i;
      while (n % i == 0)
        n /= i;
    }
  }
  if (n > 1)
    ret -= ret / n;
  return ret;
}

class Combination {
  long long powmod(long long a, long long p) {
    long long ans = 1LL;
    long long mul = a;
    while (p > 0) {
      if ((p & 1) == 1) {
        ans = (ans * mul) % mod;
      }
      mul = (mul * mul) % mod;
      p >>= 1;
    }
    return ans;
  }

public:
  int N;
  long long mod;
  vector<long long> fact;
  vector<long long> revfact;

  Combination(int n, long long m) : N(n), mod(m), fact(n + 1), revfact(n + 1) {
    fact[0] = 1;
    for (int i = 1; i <= N; i++) {
      fact[i] = fact[i - 1] * i;
      fact[i] %= mod;
    }

    revfact[N] = powmod(fact[N], mod - 2);
    for (int i = N - 1; i >= 0; i--) {
      revfact[i] = revfact[i + 1] * (i + 1) % mod;
    }
  }

  long long getCombination(int a, int b) {
    if (a < 0 || b < 0)
      return 0;
    if (b > a)
      return 0;
    return (fact[a] * revfact[b]) % mod * revfact[a - b] % mod;
  }
};

struct mint {
  const int mod = 1000000007;
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}

  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 {
    mint res(*this);
    return res += a;
  }

  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }

  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }

  mint &operator/=(const mint a) { return (*this) *= a.inv(); }

  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};

struct UnionFind {
  int n, cnt;
  vector<int> par, rank, sz;

  UnionFind(int n) : n(n), cnt(n), par(n), rank(n), sz(n, 1) {
    iota(par.begin(), par.end(), 0);
  }

  int find(int x) {
    if (x == par[x])
      return x;
    return par[x] = find(par[x]);
  }

  bool same(int x, int y) { return find(x) == find(y); }

  int size(int x) { return sz[find(x)]; }

  void unite(int x, int y) {
    x = find(x), y = find(y);
    if (x == y)
      return;
    if (rank[x] < rank[y]) {
      par[x] = y;
      sz[y] += sz[x];
    } else {
      par[y] = x;
      sz[x] += sz[y];
      if (rank[x] == rank[y]) {
        rank[x]++;
      }
    }
    cnt--;
  }
};

const string Yes = "Yes";
const string YES = "YES";
const string No = "No";
const string NO = "NO";

long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

template <typename Monoid> struct SegmentTree {
  using F = function<Monoid(Monoid, Monoid)>;

  int sz;
  vector<Monoid> seg;

  const F f;
  const Monoid M1;

  SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1) {
    sz = 1;
    while (sz < n)
      sz <<= 1;
    seg.assign(2 * sz, M1);
  }

  void set(int k, const Monoid &x) { seg[k + sz] = x; }

  void build() {
    for (int k = sz - 1; k > 0; k--) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }

  void update(int k, const Monoid &x) {
    k += sz;
    seg[k] = x;
    while (k >>= 1) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }

  Monoid query(int a, int b) {
    Monoid L = M1, R = M1;
    for (a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if (a & 1)
        L = f(L, seg[a++]);
      if (b & 1)
        R = f(seg[--b], R);
    }
    return f(L, R);
  }

  Monoid operator[](const int &k) const { return seg[k + sz]; }

  template <typename C>
  int find_subtree(int a, const C &check, Monoid &M, bool type) {
    while (a < sz) {
      Monoid nxt = type ? f(seg[2 * a + type], M) : f(M, seg[2 * a + type]);
      if (check(nxt))
        a = 2 * a + type;
      else
        M = nxt, a = 2 * a + 1 - type;
    }
    return a - sz;
  }

  template <typename C> int find_first(int a, const C &check) {
    Monoid L = M1;
    if (a <= 0) {
      if (check(f(L, seg[1])))
        return find_subtree(1, check, L, false);
      return -1;
    }
    int b = sz;
    for (a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if (a & 1) {
        Monoid nxt = f(L, seg[a]);
        if (check(nxt))
          return find_subtree(a, check, L, false);
        L = nxt;
        ++a;
      }
    }
    return -1;
  }

  template <typename C> int find_last(int b, const C &check) {
    Monoid R = M1;
    if (b >= sz) {
      if (check(f(seg[1], R)))
        return find_subtree(1, check, R, true);
      return -1;
    }
    int a = sz;
    for (b += sz; a < b; a >>= 1, b >>= 1) {
      if (b & 1) {
        Monoid nxt = f(seg[--b], R);
        if (check(nxt))
          return find_subtree(b, check, R, true);
        R = nxt;
      }
    }
    return -1;
  }
};

template <typename T> vector<T> DIVISOR(T n) {
  vector<T> v;
  for (T i = 1; i * i <= n; ++i) {
    if (n % i == 0) {
      v.push_back(i);
      if (i != n / i) {
        v.push_back(n / i);
      }
    }
  }
  sort(v.begin(), v.end());
  return v;
}

ll permutation(ll n) {
  ll ans = 1LL;
  if (n <= 0) {
    return ans;
  }
  for (ll i = 1; i <= n; i++) {
    ans *= i;
  }

  return ans;
}

template <typename T> struct PrimeFact {
  vector<T> spf;

  PrimeFact(T N) { init(N); }

  void init(T N) { // 前処理。spf を求める
    spf.assign(N + 1, 0);
    for (T i = 0; i <= N; i++)
      spf[i] = i;
    for (T i = 2; i * i <= N; i++) {
      if (spf[i] == i) {
        for (T j = i * i; j <= N; j += i) {
          if (spf[j] == j) {
            spf[j] = i;
          }
        }
      }
    }
  }

  map<T, T> get(T n) { // nの素因数分解を求める
    map<T, T> m;
    while (n != 1) {
      m[spf[n]]++;
      n /= spf[n];
    }
    return m;
  }
};

vector<int> eratosthenes(const int N) {
  std::vector<bool> is_prime(N + 1, true);
  std::vector<int> prime_vec;
  for (int i = 2; i <= N; i++) {
    if (!is_prime[i]) {
      continue;
    }
    // jは素数が確定しているのでその倍数をふるいおとす
    for (int j = 2 * i; j <= N; j += i) {
      is_prime[j] = false;
    }
    prime_vec.emplace_back(i);
  }
  return prime_vec;
}

vector<int> get_prime_divisor(const int N) {
  std::vector<bool> is_prime(N + 1, true);
  std::vector<int> prime_divisor_vec;
  for (int i = 2; i <= N; i++) {
    if (!is_prime[i]) {
      continue;
    }
    // jは素数が確定しているのでその倍数をふるいおとす
    for (int j = 2 * i; j <= N; j += i) {
      is_prime[j] = false;
    }
    if (N % i == 0) {
      prime_divisor_vec.emplace_back(i);
    }
  }
  return prime_divisor_vec;
}

int main() {
  int N;
  cin >> N;
  vector<lli> A(N);
  rep(i, N) { cin >> A[i]; }

  vector<lli> sumA(N, 0LL);
  lli sum = 0;
  for (int i = N; i > 0; i--) {
    sumA[i] = (sum + A[i]) % MOD;
    sum = (sum + A[i]) % MOD;
  }

  lli ans = 0;
  for (int i = 0; i < N - 1; i++) {
    ans = ans + (A[i] * sumA[i + 1]) % MOD;
    ans %= MOD;
  }

  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

#define BOUND 27182818284
#define MAT 2

typedef long long ll;
typedef long long int lli;
typedef pair<ll, ll> P;

ll MOD = 1000000007;
const ll INF = (1ll << 60);
const int INFint = (1 << 30);

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repi(i, a, b) for (int i = int(a); i < int(b); ++i)

template <class T> bool umax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

template <class T> bool umin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

// gcd
template <typename T> T gcd(T a, T b) {
  if (a == 0)
    return b;
  return gcd(b % a, a);
}

ll findGCD(vector<ll> arr) {
  ll result = arr[0];
  for (auto a : arr) {
    result = gcd(a, result);
  }
  return result;
}

template <typename T> T getlcm(T m, T n) {
  // 引数に0がある場合は0を返す
  if ((0 == m) || (0 == n))
    return 0;
  return ((m / gcd(m, n)) * n); // lcm = m * n / gcd(m,n)
}

template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  fill((T *)array, (T *)(array + N), val);
}

// v.front() = -BOUND;
// v.back() = BOUND;

// struct edge{
//     int cost, to;
//
//     edge(int in_cost, int in_to){
//         cost=in_cost;
//         to=in_to;
//     }
//     bool operator<(const edge &a) const
//     {
//         return cost > a.cost;
//     }
// };
ll euler_phi(ll n) {
  ll ret = n;
  for (ll i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      ret -= ret / i;
      while (n % i == 0)
        n /= i;
    }
  }
  if (n > 1)
    ret -= ret / n;
  return ret;
}

class Combination {
  long long powmod(long long a, long long p) {
    long long ans = 1LL;
    long long mul = a;
    while (p > 0) {
      if ((p & 1) == 1) {
        ans = (ans * mul) % mod;
      }
      mul = (mul * mul) % mod;
      p >>= 1;
    }
    return ans;
  }

public:
  int N;
  long long mod;
  vector<long long> fact;
  vector<long long> revfact;

  Combination(int n, long long m) : N(n), mod(m), fact(n + 1), revfact(n + 1) {
    fact[0] = 1;
    for (int i = 1; i <= N; i++) {
      fact[i] = fact[i - 1] * i;
      fact[i] %= mod;
    }

    revfact[N] = powmod(fact[N], mod - 2);
    for (int i = N - 1; i >= 0; i--) {
      revfact[i] = revfact[i + 1] * (i + 1) % mod;
    }
  }

  long long getCombination(int a, int b) {
    if (a < 0 || b < 0)
      return 0;
    if (b > a)
      return 0;
    return (fact[a] * revfact[b]) % mod * revfact[a - b] % mod;
  }
};

struct mint {
  const int mod = 1000000007;
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}

  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 {
    mint res(*this);
    return res += a;
  }

  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }

  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }

  mint &operator/=(const mint a) { return (*this) *= a.inv(); }

  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};

struct UnionFind {
  int n, cnt;
  vector<int> par, rank, sz;

  UnionFind(int n) : n(n), cnt(n), par(n), rank(n), sz(n, 1) {
    iota(par.begin(), par.end(), 0);
  }

  int find(int x) {
    if (x == par[x])
      return x;
    return par[x] = find(par[x]);
  }

  bool same(int x, int y) { return find(x) == find(y); }

  int size(int x) { return sz[find(x)]; }

  void unite(int x, int y) {
    x = find(x), y = find(y);
    if (x == y)
      return;
    if (rank[x] < rank[y]) {
      par[x] = y;
      sz[y] += sz[x];
    } else {
      par[y] = x;
      sz[x] += sz[y];
      if (rank[x] == rank[y]) {
        rank[x]++;
      }
    }
    cnt--;
  }
};

const string Yes = "Yes";
const string YES = "YES";
const string No = "No";
const string NO = "NO";

long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

template <typename Monoid> struct SegmentTree {
  using F = function<Monoid(Monoid, Monoid)>;

  int sz;
  vector<Monoid> seg;

  const F f;
  const Monoid M1;

  SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1) {
    sz = 1;
    while (sz < n)
      sz <<= 1;
    seg.assign(2 * sz, M1);
  }

  void set(int k, const Monoid &x) { seg[k + sz] = x; }

  void build() {
    for (int k = sz - 1; k > 0; k--) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }

  void update(int k, const Monoid &x) {
    k += sz;
    seg[k] = x;
    while (k >>= 1) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }

  Monoid query(int a, int b) {
    Monoid L = M1, R = M1;
    for (a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if (a & 1)
        L = f(L, seg[a++]);
      if (b & 1)
        R = f(seg[--b], R);
    }
    return f(L, R);
  }

  Monoid operator[](const int &k) const { return seg[k + sz]; }

  template <typename C>
  int find_subtree(int a, const C &check, Monoid &M, bool type) {
    while (a < sz) {
      Monoid nxt = type ? f(seg[2 * a + type], M) : f(M, seg[2 * a + type]);
      if (check(nxt))
        a = 2 * a + type;
      else
        M = nxt, a = 2 * a + 1 - type;
    }
    return a - sz;
  }

  template <typename C> int find_first(int a, const C &check) {
    Monoid L = M1;
    if (a <= 0) {
      if (check(f(L, seg[1])))
        return find_subtree(1, check, L, false);
      return -1;
    }
    int b = sz;
    for (a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if (a & 1) {
        Monoid nxt = f(L, seg[a]);
        if (check(nxt))
          return find_subtree(a, check, L, false);
        L = nxt;
        ++a;
      }
    }
    return -1;
  }

  template <typename C> int find_last(int b, const C &check) {
    Monoid R = M1;
    if (b >= sz) {
      if (check(f(seg[1], R)))
        return find_subtree(1, check, R, true);
      return -1;
    }
    int a = sz;
    for (b += sz; a < b; a >>= 1, b >>= 1) {
      if (b & 1) {
        Monoid nxt = f(seg[--b], R);
        if (check(nxt))
          return find_subtree(b, check, R, true);
        R = nxt;
      }
    }
    return -1;
  }
};

template <typename T> vector<T> DIVISOR(T n) {
  vector<T> v;
  for (T i = 1; i * i <= n; ++i) {
    if (n % i == 0) {
      v.push_back(i);
      if (i != n / i) {
        v.push_back(n / i);
      }
    }
  }
  sort(v.begin(), v.end());
  return v;
}

ll permutation(ll n) {
  ll ans = 1LL;
  if (n <= 0) {
    return ans;
  }
  for (ll i = 1; i <= n; i++) {
    ans *= i;
  }

  return ans;
}

template <typename T> struct PrimeFact {
  vector<T> spf;

  PrimeFact(T N) { init(N); }

  void init(T N) { // 前処理。spf を求める
    spf.assign(N + 1, 0);
    for (T i = 0; i <= N; i++)
      spf[i] = i;
    for (T i = 2; i * i <= N; i++) {
      if (spf[i] == i) {
        for (T j = i * i; j <= N; j += i) {
          if (spf[j] == j) {
            spf[j] = i;
          }
        }
      }
    }
  }

  map<T, T> get(T n) { // nの素因数分解を求める
    map<T, T> m;
    while (n != 1) {
      m[spf[n]]++;
      n /= spf[n];
    }
    return m;
  }
};

vector<int> eratosthenes(const int N) {
  std::vector<bool> is_prime(N + 1, true);
  std::vector<int> prime_vec;
  for (int i = 2; i <= N; i++) {
    if (!is_prime[i]) {
      continue;
    }
    // jは素数が確定しているのでその倍数をふるいおとす
    for (int j = 2 * i; j <= N; j += i) {
      is_prime[j] = false;
    }
    prime_vec.emplace_back(i);
  }
  return prime_vec;
}

vector<int> get_prime_divisor(const int N) {
  std::vector<bool> is_prime(N + 1, true);
  std::vector<int> prime_divisor_vec;
  for (int i = 2; i <= N; i++) {
    if (!is_prime[i]) {
      continue;
    }
    // jは素数が確定しているのでその倍数をふるいおとす
    for (int j = 2 * i; j <= N; j += i) {
      is_prime[j] = false;
    }
    if (N % i == 0) {
      prime_divisor_vec.emplace_back(i);
    }
  }
  return prime_divisor_vec;
}

int main() {
  int N;
  cin >> N;
  vector<lli> A(N);
  rep(i, N) { cin >> A[i]; }

  vector<lli> sumA(N, 0LL);
  lli sum = 0;
  for (int i = N - 1; i > 0; i--) {
    sumA[i] = (sum + A[i]) % MOD;
    sum = (sum + A[i]) % MOD;
  }

  lli ans = 0;
  for (int i = 0; i < N - 1; i++) {
    ans = ans + (A[i] * sumA[i + 1]) % MOD;
    ans %= MOD;
  }

  cout << ans << endl;
  return 0;
}