Problem p02572 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for (int i = (a); i < (int)(b); ++i)
#define endl "\\n"
typedef long long ll;
typedef string str;
const double pi = 3.14159265358979323846;
const long long inf = 1LL << 60;

int ctoi(const char c) {
  if ('0' <= c && c <= '9')
    return (c - '0');
  return -1;
}
vector<int> input(int n) {
  vector<int> vec(n);
  for (int i = 0; i < n; i++) {
    cin >> vec.at(i);
  }
  return vec;
}
vector<vector<int>> matinput(int n, int m) {
  vector<vector<int>> table(n, vector<int>(m));
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      cin >> table.at(i).at(j);
    }
  }
  return table;
}
void matoutput(vector<vector<int>> table) {
  for (int i = 0; i < int(table.size()); i++) {
    for (int j = 0; j < int(table.at(0).size()); j++) {
      cout << table.at(i).at(j) << " ";
    }
    cout << endl;
  }
}
long long perm(int n, int r) {
  if (n < r) {
    cout << "error" << endl;
    return 0;
  }
  long long perm = 1;
  for (int i = n; i > n - r; i--) {
    perm *= i;
  }
  return perm;
}
long long comb(int n, int r) {
  if (n < r) {
    cout << "error" << endl;
    return 0;
  }
  long long comb = perm(n, r);
  for (int i = r; i > 0; i--) {
    comb /= i;
  }
  return comb;
}
long long fact(int n) {
  long long fact = 1;
  for (int i = n; i > 0; i--) {
    fact *= i;
  }
  return fact;
}
int gcd(int a, int b) {
  if (a % b == 0) {
    return (b);
  } else {
    return (gcd(b, a % b));
  }
}
int lcm(int a, int b) { return a * b / gcd(a, b); }
bool isprime(int n) {
  if (n < 2)
    return false;
  else if (n == 2)
    return true;
  else if (n % 2 == 0)
    return false;

  for (int i = 3; i <= sqrt(n); i += 2) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}
vector<long long> divisors(long long N) {
  vector<long long> res;
  for (long long i = 1; i * i <= N; ++i) {
    if (N % i == 0) {
      res.push_back(i);
      // 重複しないならば i の相方である N/i も push
      if (N / i != i)
        res.push_back(N / i);
    }
  }
  // 小さい順に並び替える
  sort(res.begin(), res.end());
  return res;

  /*long long N;
  cin >> N;
  vector<long long> res = divisors(N);
  for (int i = 0; i < res.size(); ++i) {
      cout << res[i] << " ";
  }
  cout << endl;*/
}
vector<pair<long long, long long>> prime_factorize(long long N) {
  vector<pair<long long, long long>> res;
  for (long long a = 2; a * a <= N; ++a) {
    if (N % a != 0)
      continue;
    long long ex = 0;
    while (N % a == 0) {
      ++ex;
      N /= a;
    }
    res.push_back({a, ex});
  }
  if (N != 1)
    res.push_back({N, 1});
  return res;
}
void Yes(bool f) {
  if (f) {
    cout << "Yes" << endl;
  } else {
    cout << "No" << endl;
  }
}
void YES(bool f) {
  if (f) {
    cout << "YES" << endl;
  } else {
    cout << "NO" << endl;
  }
}
bool leapyear(int y) {
  // 閏年かどうかを判定
  if (y % 4 == 0) {
    if (y % 100 == 0) {
      if (y % 400 == 0) {
        return true;
      } else {
        return false;
      }
    } else {
      return true;
    }
  } else {
    return false;
  }
}
tuple<int, int, int> dayplus(int y, int m, int d) {
  // 次の日を返す
  // 31日まである月
  if (m == 1 || m == 3 || m == 5 || m == 7 || m == 8 || m == 10 || m == 12) {
    if (d != 31) {
      d++;
      return make_tuple(y, m, d);
    } else {
      d = 1;
      if (m == 12) {
        m = 1;
        y++;
        return make_tuple(y, m, d);
      } else {
        m++;
        return make_tuple(y, m, d);
      }
    }
  }
  // 30日まである月
  else if (m == 4 || m == 6 || m == 9 || m == 11) {
    if (d != 30) {
      d++;
      return make_tuple(y, m, d);
    } else {
      d = 1;
      m++;
      return make_tuple(y, m, d);
    }
  }
  // 2月
  else {
    // 閏年の場合
    if (leapyear(y)) {
      if (d != 29) {
        d++;
        return make_tuple(y, m, d);
      } else {
        d = 1;
        m++;
        return make_tuple(y, m, d);
      }
    }
    // 閏年ではない場合
    else {
      if (d != 28) {
        d++;
        return make_tuple(y, m, d);
      } else {
        d = 1;
        m++;
        return make_tuple(y, m, d);
      }
    }
  }
}
tuple<int, int, int> monplus(int y, int m, int d) {
  // 月を一つ先に、日を一日にする
  d = 1;
  if (m == 12) {
    m = 1;
    y++;
    return make_tuple(y, m, d);
  } else {
    m++;
    return make_tuple(y, m, d);
  }
}
bool in(vector<int> a, int x) {
  if (find(a.begin(), a.end(), x) != a.end()) {
    return true;
  } else {
    return false;
  }
}
int count_bool(vector<bool> flag) {
  int count = 0;
  for (int i = 0; i < int(flag.size()); i++) {
    if (flag[i]) {
      count++;
    }
  }
  return count;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const ll mod = 1000000007;
int main() {
  int n;
  cin >> n;
  ll ans, a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    ans += a[i];
    ans %= mod;
  }
  ans = ans * ans % mod;
  for (int i = 0; i < n; i++) {
    ans -= a[i] * a[i] % mod;
    if (ans < 0) {
      ans += mod;
    }
  }
  ans *= (mod + 1) / 2;
  ans %= mod;
  cout << ans << endl;
}


Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for (int i = (a); i < (int)(b); ++i)
#define endl "\\n"
typedef long long ll;
typedef string str;
const double pi = 3.14159265358979323846;
const long long inf = 1LL << 60;

int ctoi(const char c) {
  if ('0' <= c && c <= '9')
    return (c - '0');
  return -1;
}
vector<int> input(int n) {
  vector<int> vec(n);
  for (int i = 0; i < n; i++) {
    cin >> vec.at(i);
  }
  return vec;
}
vector<vector<int>> matinput(int n, int m) {
  vector<vector<int>> table(n, vector<int>(m));
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      cin >> table.at(i).at(j);
    }
  }
  return table;
}
void matoutput(vector<vector<int>> table) {
  for (int i = 0; i < int(table.size()); i++) {
    for (int j = 0; j < int(table.at(0).size()); j++) {
      cout << table.at(i).at(j) << " ";
    }
    cout << endl;
  }
}
long long perm(int n, int r) {
  if (n < r) {
    cout << "error" << endl;
    return 0;
  }
  long long perm = 1;
  for (int i = n; i > n - r; i--) {
    perm *= i;
  }
  return perm;
}
long long comb(int n, int r) {
  if (n < r) {
    cout << "error" << endl;
    return 0;
  }
  long long comb = perm(n, r);
  for (int i = r; i > 0; i--) {
    comb /= i;
  }
  return comb;
}
long long fact(int n) {
  long long fact = 1;
  for (int i = n; i > 0; i--) {
    fact *= i;
  }
  return fact;
}
int gcd(int a, int b) {
  if (a % b == 0) {
    return (b);
  } else {
    return (gcd(b, a % b));
  }
}
int lcm(int a, int b) { return a * b / gcd(a, b); }
bool isprime(int n) {
  if (n < 2)
    return false;
  else if (n == 2)
    return true;
  else if (n % 2 == 0)
    return false;

  for (int i = 3; i <= sqrt(n); i += 2) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}
vector<long long> divisors(long long N) {
  vector<long long> res;
  for (long long i = 1; i * i <= N; ++i) {
    if (N % i == 0) {
      res.push_back(i);
      // 重複しないならば i の相方である N/i も push
      if (N / i != i)
        res.push_back(N / i);
    }
  }
  // 小さい順に並び替える
  sort(res.begin(), res.end());
  return res;

  /*long long N;
  cin >> N;
  vector<long long> res = divisors(N);
  for (int i = 0; i < res.size(); ++i) {
      cout << res[i] << " ";
  }
  cout << endl;*/
}
vector<pair<long long, long long>> prime_factorize(long long N) {
  vector<pair<long long, long long>> res;
  for (long long a = 2; a * a <= N; ++a) {
    if (N % a != 0)
      continue;
    long long ex = 0;
    while (N % a == 0) {
      ++ex;
      N /= a;
    }
    res.push_back({a, ex});
  }
  if (N != 1)
    res.push_back({N, 1});
  return res;
}
void Yes(bool f) {
  if (f) {
    cout << "Yes" << endl;
  } else {
    cout << "No" << endl;
  }
}
void YES(bool f) {
  if (f) {
    cout << "YES" << endl;
  } else {
    cout << "NO" << endl;
  }
}
bool leapyear(int y) {
  // 閏年かどうかを判定
  if (y % 4 == 0) {
    if (y % 100 == 0) {
      if (y % 400 == 0) {
        return true;
      } else {
        return false;
      }
    } else {
      return true;
    }
  } else {
    return false;
  }
}
tuple<int, int, int> dayplus(int y, int m, int d) {
  // 次の日を返す
  // 31日まである月
  if (m == 1 || m == 3 || m == 5 || m == 7 || m == 8 || m == 10 || m == 12) {
    if (d != 31) {
      d++;
      return make_tuple(y, m, d);
    } else {
      d = 1;
      if (m == 12) {
        m = 1;
        y++;
        return make_tuple(y, m, d);
      } else {
        m++;
        return make_tuple(y, m, d);
      }
    }
  }
  // 30日まである月
  else if (m == 4 || m == 6 || m == 9 || m == 11) {
    if (d != 30) {
      d++;
      return make_tuple(y, m, d);
    } else {
      d = 1;
      m++;
      return make_tuple(y, m, d);
    }
  }
  // 2月
  else {
    // 閏年の場合
    if (leapyear(y)) {
      if (d != 29) {
        d++;
        return make_tuple(y, m, d);
      } else {
        d = 1;
        m++;
        return make_tuple(y, m, d);
      }
    }
    // 閏年ではない場合
    else {
      if (d != 28) {
        d++;
        return make_tuple(y, m, d);
      } else {
        d = 1;
        m++;
        return make_tuple(y, m, d);
      }
    }
  }
}
tuple<int, int, int> monplus(int y, int m, int d) {
  // 月を一つ先に、日を一日にする
  d = 1;
  if (m == 12) {
    m = 1;
    y++;
    return make_tuple(y, m, d);
  } else {
    m++;
    return make_tuple(y, m, d);
  }
}
bool in(vector<int> a, int x) {
  if (find(a.begin(), a.end(), x) != a.end()) {
    return true;
  } else {
    return false;
  }
}
int count_bool(vector<bool> flag) {
  int count = 0;
  for (int i = 0; i < int(flag.size()); i++) {
    if (flag[i]) {
      count++;
    }
  }
  return count;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T>
bool next_combination(const T first, const T last, int k) {
  const T subset = first + k;
  // empty container | k = 0 | k == n
  if (first == last || first == subset || last == subset) {
    return false;
  }
  T src = subset;
  while (first != src) {
    src--;
    if (*src < *(last - 1)) {
      T dest = subset;
      while (*src >= *dest) {
        dest++;
      }
      iter_swap(src, dest);
      rotate(src + 1, dest + 1, last);
      rotate(subset, subset + (last - dest) - 1, last);
      return true;
    }
  }
  // restore
  rotate(first, subset, last);
  return false;
}
const ll mod = 1000000007;
int main() {
  ll n, sum = 0;
  cin >> n;
  vector<ll> v(n), a(n);
  rep(i, 0, n) {
    cin >> a[i];
    v[i] = i;
  }

  do {
    sum += (a[v[0]] * a[v[1]]) % mod;
    sum %= mod;
  } while (next_combination(v.begin(), v.end(), 2));
  cout << sum << endl;
}