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

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
#define REP(i, m, n) for (ll i = (ll)m; i < (ll)n; i++)
#define PER(i, m, n) for (ll i = (ll)m; i > (ll)n; i--)
#define INF 1e9
#define ALL(v) v.begin(), v.end()
#define _GLIBCXX_DEBUG

template <uint MD> struct ModInt {
  using M = ModInt;
  const static M G;
  uint v;
  ModInt(ll _v = 0) { set_v(uint(_v % MD + MD)); }
  M &set_v(uint _v) {
    v = (_v < MD) ? _v : _v - MD;
    return *this;
  }
  explicit operator bool() const { return v != 0; }
  M operator-() const { return M() - *this; }
  M operator+(const M &r) const { return M().set_v(v + r.v); }
  M operator-(const M &r) const { return M().set_v(v + MD - r.v); }
  M operator*(const M &r) const { return M().set_v(uint(ull(v) * r.v % MD)); }
  M operator/(const M &r) const { return *this * r.inv(); }
  M &operator+=(const M &r) { return *this = *this + r; }
  M &operator-=(const M &r) { return *this = *this - r; }
  M &operator*=(const M &r) { return *this = *this * r; }
  M &operator/=(const M &r) { return *this = *this / r; }
  bool operator==(const M &r) const { return v == r.v; }
  M pow(ll n) const {
    M x = *this, r = 1;
    while (n) {
      if (n & 1)
        r *= x;
      x *= x;
      n >>= 1;
    }
    return r;
  }
  M inv() const { return pow(MD - 2); }
  friend ostream &operator<<(ostream &os, const M &r) { return os << r.v; }
};
using Mint = ModInt<1000000007>;

const double PI = 3.14159265358979323846;

bool compare_by_b(pair<int, int> a, pair<int, int> b) {
  if (a.second != b.second) {
    return a.second < b.second;
  } else {
    return a.first < b.first;
  }
}

bool is_prime(long long N) {
  if (N == 1)
    return false;
  for (long long i = 2; i * i <= N; ++i) {
    if (N % i == 0)
      return false;
  }
  return true;
}

int main() {
  ll n;
  cin >> n;
  vector<ll> v(n);
  REP(i, 0, n) cin >> v[i];
  Mint ans = 0;
  Mint sum = 0;

  PER(i, n - 1, 0) {
    sum += v[i];
    ans += sum * v[i - 1];
  }

  cout << ans << endl;
}


Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
#define REP(i, m, n) for (ll i = (ll)m; i < (ll)n; i++)
#define PER(i, m, n) for (ll i = (ll)m; i > (ll)n; i--)
#define INF 1e9
#define ALL(v) v.begin(), v.end()
#define _GLIBCXX_DEBUG

template <uint MD> struct ModInt {
  using M = ModInt;
  const static M G;
  uint v;
  ModInt(ll _v = 0) { set_v(uint(_v % MD + MD)); }
  M &set_v(uint _v) {
    v = (_v < MD) ? _v : _v - MD;
    return *this;
  }
  explicit operator bool() const { return v != 0; }
  M operator-() const { return M() - *this; }
  M operator+(const M &r) const { return M().set_v(v + r.v); }
  M operator-(const M &r) const { return M().set_v(v + MD - r.v); }
  M operator*(const M &r) const { return M().set_v(uint(ull(v) * r.v % MD)); }
  M operator/(const M &r) const { return *this * r.inv(); }
  M &operator+=(const M &r) { return *this = *this + r; }
  M &operator-=(const M &r) { return *this = *this - r; }
  M &operator*=(const M &r) { return *this = *this * r; }
  M &operator/=(const M &r) { return *this = *this / r; }
  bool operator==(const M &r) const { return v == r.v; }
  M pow(ll n) const {
    M x = *this, r = 1;
    while (n) {
      if (n & 1)
        r *= x;
      x *= x;
      n >>= 1;
    }
    return r;
  }
  M inv() const { return pow(MD - 2); }
  friend ostream &operator<<(ostream &os, const M &r) { return os << r.v; }
};
using Mint = ModInt<1000000007>;

const double PI = 3.14159265358979323846;

bool compare_by_b(pair<int, int> a, pair<int, int> b) {
  if (a.second != b.second) {
    return a.second < b.second;
  } else {
    return a.first < b.first;
  }
}

bool is_prime(long long N) {
  if (N == 1)
    return false;
  for (long long i = 2; i * i <= N; ++i) {
    if (N % i == 0)
      return false;
  }
  return true;
}

int main() {
  ll n;
  cin >> n;
  vector<ll> v(n);
  REP(i, 0, n) cin >> v[i];
  Mint ans = 0;

  REP(i, 0, n - 1) {
    REP(j, i + 1, n) { ans += v[i] * v[j]; }
  }

  cout << ans << endl;
}