Problem p02624 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll, ll>;
using Pld = pair<ld, ld>;
using Vec = vector<ll>;
using VecP = vector<P>;
using VecB = vector<bool>;
using VecC = vector<char>;
using VecD = vector<ld>;
using VecS = vector<string>;
using Graph = vector<VecP>;
template <typename T> using Vec1 = vector<T>;
template <typename T> using Vec2 = vector<Vec1<T>>;
#define REP(i, m, n) for (ll i = (m); i < (n); ++i)
#define REPN(i, m, n) for (ll i = (m); i <= (n); ++i)
#define REPR(i, m, n) for (ll i = (m)-1; i >= (n); --i)
#define REPNR(i, m, n) for (ll i = (m); i >= (n); --i)
#define rep(i, n) REP(i, 0, n)
#define repn(i, n) REPN(i, 1, n)
#define repr(i, n) REPR(i, n, 0)
#define repnr(i, n) REPNR(i, n, 1)
#define all(s) (s).begin(), (s).end()
#define pb push_back
#define fs first
#define sc second
template <typename T> bool chmax(T &a, const T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, const T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ll pow2(const T n) { return (1LL << n); }
template <typename T> void cosp(const T n) { cout << n << ' '; }
void co(void) { cout << '\n'; }
template <typename T> void co(const T n) { cout << n << '\n'; }
template <typename T1, typename T2> void co(pair<T1, T2> p) {
  cout << p.fs << ' ' << p.sc << '\n';
}
template <typename T> void co(const Vec1<T> &v) {
  for (T i : v)
    cosp(i);
  co();
}
template <typename T> void co(initializer_list<T> v) {
  for (T i : v)
    cosp(i);
  co();
}
template <typename T> void ce(const T n) { cerr << n << endl; }
void sonic() {
  ios::sync_with_stdio(false);
  cin.tie(0);
}
void setp(const ll n) { cout << fixed << setprecision(n); }
constexpr int INF = 1e9 + 1;
constexpr ll LINF = 1e18 + 1;
constexpr ll MOD = 1e9 + 7;
// constexpr ll MOD = 998244353;
constexpr ld EPS = 1e-11;
const ld PI = acos(-1);

using VecP = Vec1<P>;

Vec pn;

bool isPrime(ll n) {
  if (n == 1)
    return false;
  for (ll i : pn) {
    if (i * i > n)
      return true;
    if (n % i == 0)
      return false;
  }
  return true;
}

void init(ll n = 2e5) {
  pn.pb(2);
  REP(num, 3, n) if (isPrime(num)) pn.pb(num);
}

VecP primeFactorization(ll n) {
  VecP res;

  for (ll i : pn) {
    ll cnt = 0;
    while (n % i == 0) {
      n /= i;
      cnt++;
    }
    if (cnt)
      res.pb({i, cnt});
    if (n < i * i)
      break;
  }
  if (n != 1)
    res.pb({n, 1});

  return res;
}

Vec divisorEnumeration(ll n) {
  auto v = primeFactorization(n);

  Vec res, a, b, cp;
  res.pb(1);
  for (auto p : v) {
    cp.resize(res.size());
    copy(all(res), cp.begin());
    a.resize(res.size());
    repn(k, p.sc) {
      ll t = pow(p.fs, k);
      rep(i, a.size()) a[i] = cp[i] * t;
      merge(all(res), all(a), back_inserter(b));
      swap(res, b);
      b.clear();
    }
  }

  return res;
}

int main(void) {
  init();
  ll n;
  cin >> n;

  Vec f(n + 1, 1);
  for (ll i = 2; i <= n; ++i) {
    for (ll j = i; j <= n; ++j)
      if (j % i == 0)
        ++f[j];
  }

  ll ans = 0;
  repn(i, n) ans += i * f[i];
  co(ans);

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll, ll>;
using Pld = pair<ld, ld>;
using Vec = vector<ll>;
using VecP = vector<P>;
using VecB = vector<bool>;
using VecC = vector<char>;
using VecD = vector<ld>;
using VecS = vector<string>;
using Graph = vector<VecP>;
template <typename T> using Vec1 = vector<T>;
template <typename T> using Vec2 = vector<Vec1<T>>;
#define REP(i, m, n) for (ll i = (m); i < (n); ++i)
#define REPN(i, m, n) for (ll i = (m); i <= (n); ++i)
#define REPR(i, m, n) for (ll i = (m)-1; i >= (n); --i)
#define REPNR(i, m, n) for (ll i = (m); i >= (n); --i)
#define rep(i, n) REP(i, 0, n)
#define repn(i, n) REPN(i, 1, n)
#define repr(i, n) REPR(i, n, 0)
#define repnr(i, n) REPNR(i, n, 1)
#define all(s) (s).begin(), (s).end()
#define pb push_back
#define fs first
#define sc second
template <typename T> bool chmax(T &a, const T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, const T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ll pow2(const T n) { return (1LL << n); }
template <typename T> void cosp(const T n) { cout << n << ' '; }
void co(void) { cout << '\n'; }
template <typename T> void co(const T n) { cout << n << '\n'; }
template <typename T1, typename T2> void co(pair<T1, T2> p) {
  cout << p.fs << ' ' << p.sc << '\n';
}
template <typename T> void co(const Vec1<T> &v) {
  for (T i : v)
    cosp(i);
  co();
}
template <typename T> void co(initializer_list<T> v) {
  for (T i : v)
    cosp(i);
  co();
}
template <typename T> void ce(const T n) { cerr << n << endl; }
void sonic() {
  ios::sync_with_stdio(false);
  cin.tie(0);
}
void setp(const ll n) { cout << fixed << setprecision(n); }
constexpr int INF = 1e9 + 1;
constexpr ll LINF = 1e18 + 1;
constexpr ll MOD = 1e9 + 7;
// constexpr ll MOD = 998244353;
constexpr ld EPS = 1e-11;
const ld PI = acos(-1);

using VecP = Vec1<P>;

Vec pn;

bool isPrime(ll n) {
  if (n == 1)
    return false;
  for (ll i : pn) {
    if (i * i > n)
      return true;
    if (n % i == 0)
      return false;
  }
  return true;
}

void init(ll n = 2e5) {
  pn.pb(2);
  REP(num, 3, n) if (isPrime(num)) pn.pb(num);
}

VecP primeFactorization(ll n) {
  VecP res;

  for (ll i : pn) {
    ll cnt = 0;
    while (n % i == 0) {
      n /= i;
      cnt++;
    }
    if (cnt)
      res.pb({i, cnt});
    if (n < i * i)
      break;
  }
  if (n != 1)
    res.pb({n, 1});

  return res;
}

Vec divisorEnumeration(ll n) {
  auto v = primeFactorization(n);

  Vec res, a, b, cp;
  res.pb(1);
  for (auto p : v) {
    cp.resize(res.size());
    copy(all(res), cp.begin());
    a.resize(res.size());
    repn(k, p.sc) {
      ll t = pow(p.fs, k);
      rep(i, a.size()) a[i] = cp[i] * t;
      merge(all(res), all(a), back_inserter(b));
      swap(res, b);
      b.clear();
    }
  }

  return res;
}

int main(void) {
  init();
  ll n;
  cin >> n;

  Vec f(n + 1, 1);
  for (ll i = 2; i <= n; ++i) {
    for (ll j = i; j <= n; j += i)
      ++f[j];
  }

  ll ans = 0;
  repn(i, n) ans += i * f[i];
  co(ans);

  return 0;
}