Problem p03762 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rrep(i, n) for (int i = n - 1; i >= 0; i--)
#define rep2(i, s, n) for (int i = s; i < n; ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define tmax(a, b, c) max(a, max(b, c))
#define tmin(a, b, c) min(a, min(b, c))
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<P>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (T t : v) {
    os << t << " ";
  }
  os << "\n";
  return os;
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YES(bool b) { cout << (b ? "YES" : "NO") << endl; }
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 int inf = 1001001001;
const ll linf = 1001001001001001001;

const int mod = 1000000007;
struct mint {
  ll x;
  mint(ll 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++() {
    x += 1;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator--() {
    x -= 1;
    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;
  }
  bool operator==(const int n) const {
    mint a(n);
    return this->x == a.x;
  }
  bool operator==(const ll n) const {
    mint a(n);
    return this->x == a.x;
  }
  bool operator!=(const int n) const {
    mint a(n);
    return this->x != a.x;
  }
  bool operator!=(const ll n) const {
    mint a(n);
    return this->x != a.x;
  }
  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;
  }
};
istream &operator>>(istream &is, const mint &m) { return is >> m.x; }
ostream &operator<<(ostream &os, const mint &m) { return os << m.x; }

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int n, m;
  cin >> n >> m;
  vi x(n), y(m);
  cin >> x >> y;
  mint ans = 0;
  rep(i, n - 1) rep(j, m - 1) {
    mint nans = (ll)(x[i + 1] - x[i]) * (y[j + 1] - y[j]);
    nans *= i + 1;
    nans *= n - 1 - i;
    nans *= j + 1;
    nans *= m - 1 - j;
    ans += nans;
  }
  cout << ans << endl;
}


Fixed submission:
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rrep(i, n) for (int i = n - 1; i >= 0; i--)
#define rep2(i, s, n) for (int i = s; i < n; ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define tmax(a, b, c) max(a, max(b, c))
#define tmin(a, b, c) min(a, min(b, c))
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<P>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (T t : v) {
    os << t << " ";
  }
  os << "\n";
  return os;
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YES(bool b) { cout << (b ? "YES" : "NO") << endl; }
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 int inf = 1001001001;
const ll linf = 1001001001001001001;

const int mod = 1000000007;
struct mint {
  ll x;
  mint(ll 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++() {
    x += 1;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator--() {
    x -= 1;
    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;
  }
  bool operator==(const int n) const {
    mint a(n);
    return this->x == a.x;
  }
  bool operator==(const ll n) const {
    mint a(n);
    return this->x == a.x;
  }
  bool operator!=(const int n) const {
    mint a(n);
    return this->x != a.x;
  }
  bool operator!=(const ll n) const {
    mint a(n);
    return this->x != a.x;
  }
  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;
  }
};
istream &operator>>(istream &is, const mint &m) { return is >> m.x; }
ostream &operator<<(ostream &os, const mint &m) { return os << m.x; }

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int n, m;
  cin >> n >> m;
  vi x(n), y(m);
  cin >> x >> y;
  mint ansx = 0, ansy = 0;
  rep(i, n - 1) ansx += mint(x[i + 1] - x[i]) * mint(i + 1) * mint(n - 1 - i);
  rep(i, m - 1) ansy += mint(y[i + 1] - y[i]) * mint(i + 1) * mint(m - 1 - i);
  cout << ansx * ansy << endl;
}