Problem p02624 (C++)

Buggy submission (status: Time Limit Exceeded):
// K-OS WITH THE OCDE

#include <bits/stdc++.h>
using namespace std;
#define ll long long int
#define ff first
#define ss second
#define ar array
#define mod 1000000007
#define pb push_back
#define mpa make_pair
#define TEST                                                                   \
  ll t;                                                                        \
  cin >> t;                                                                    \
  while (t--)
#define vell vector<ll>
#define vel2 vector<vector<ll>>
#define all(v) v.begin(), v.end()

#define for0(i, n) for (ll i = 0; i < (ll)n; i++)
#define for1(i, n) for (ll i = 1; i <= (ll)n; i++)
#define forr(i, n) for (ll i = (ll)n - 1; i >= 0; i--)
#define fall(i, a, b, c) for (ll i = a; i <= (ll)b; i += (ll)c)

#define db long double
#define endl "\n"
#define MAX INT_MAX
#define MIN INT_MIN

////////////////////////////////////////////////////////////////////////

/*Parity and inversion - ( (grid width odd) && (#inversions even) ) ||
( (grid width even) && ((blank on odd row from bottom) == (#inversions even))
)*/

////////////////////////////////////////////////////////////////////////

ll add(ll a, ll b) { return ((a % mod + b % mod) % mod); }
ll mult(ll a, ll b) { return (((a % mod) * (b % mod)) % mod); }

ll gcdExtended(ll a, ll b, ll *x, ll *y) {
  if (a == 0) {
    *x = 0, *y = 1;
    return b;
  }

  ll x1, y1;
  ll gcd = gcdExtended(b % a, a, &x1, &y1);
  *x = y1 - (b / a) * x1;
  *y = x1;
  return gcd;
}

ll modInverse(ll b, ll m) {
  ll x, y;
  ll g = gcdExtended(b, m, &x, &y);
  if (g != 1)
    return -1;
  return (x % m + m) % m;
}

ll modDivide(ll a, ll b, ll m) {
  a = a % m;
  int inv = modInverse(b, m);
  return (inv * a) % m;
}

ll power(ll a, ll n) {
  ll ans = 1;
  while (n > 0) {
    ll lastbit = (n & 1);
    if (lastbit)
      ans *= a;

    a *= a;
    n >>= 1;
  }

  return ans;
}
/////////////////////////////////////////////////
int sieveo(ll n) {

  bool prime[n + 1];
  memset(prime, true, sizeof(prime));
  ll ans = 0;

  for (ll p = 2; p * p <= n; p++) {
    if (prime[p] == true) {

      for (ll i = p * p; i <= n; i += p)
        prime[i] = false;
    }
  }

  for (ll p = 2; p <= n; p++)
    if (prime[p])
      ans++;

  return ans;
}

///////////////////////////////

void primesieve(ll N, ll s[]) {
  vector<bool> prime(N + 1, false);

  for (ll i = 2; i <= N; i += 2)
    s[i] = 2;

  for (ll i = 3; i <= N; i += 2) {
    if (prime[i] == false) {
      s[i] = i;

      for (ll j = i; j * i <= N; j += 2) {
        if (prime[i * j] == false) {
          prime[i * j] = true;
          s[i * j] = i;
        }
      }
    }
  }
}

void primefactor(ll N) {

  ll s[N + 1];
  primesieve(N, s);

  ll curr = s[N];
  ll cnt = 1;

  while (N > 1) {
    N /= s[N];
    if (curr == s[N]) {
      cnt++;
      continue;
    }

    // curr==prime n.
    // cnt=power
    curr = s[N];
    cnt = 1;
  }
}

/////////////////////////////////////////////////////////

void solve() {
  ll n;
  cin >> n;
  ll sum = 0;
  for (ll i = 1; i <= n; ++i) {
    ll c = 0;
    for (ll j = 1; j * j <= i; ++j) {
      if (i % j == 0)
        c += (i / j == j ? 1 : 2);
    }
    sum += i * c;
  }

  cout << sum;
}

int main() {
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  // TEST
  solve();
}


Fixed submission:
// K-OS WITH THE OCDE

#include <bits/stdc++.h>
using namespace std;
#define ll long long int
#define ff first
#define ss second
#define ar array
#define mod 1000000007
#define pb push_back
#define mpa make_pair
#define TEST                                                                   \
  ll t;                                                                        \
  cin >> t;                                                                    \
  while (t--)
#define vell vector<ll>
#define vel2 vector<vector<ll>>
#define all(v) v.begin(), v.end()

#define for0(i, n) for (ll i = 0; i < (ll)n; i++)
#define for1(i, n) for (ll i = 1; i <= (ll)n; i++)
#define forr(i, n) for (ll i = (ll)n - 1; i >= 0; i--)
#define fall(i, a, b, c) for (ll i = a; i <= (ll)b; i += (ll)c)

#define db long double
#define endl "\n"
#define MAX INT_MAX
#define MIN INT_MIN

////////////////////////////////////////////////////////////////////////

/*Parity and inversion - ( (grid width odd) && (#inversions even) ) ||
( (grid width even) && ((blank on odd row from bottom) == (#inversions even))
)*/

////////////////////////////////////////////////////////////////////////

ll add(ll a, ll b) { return ((a % mod + b % mod) % mod); }
ll mult(ll a, ll b) { return (((a % mod) * (b % mod)) % mod); }

ll gcdExtended(ll a, ll b, ll *x, ll *y) {
  if (a == 0) {
    *x = 0, *y = 1;
    return b;
  }

  ll x1, y1;
  ll gcd = gcdExtended(b % a, a, &x1, &y1);
  *x = y1 - (b / a) * x1;
  *y = x1;
  return gcd;
}

ll modInverse(ll b, ll m) {
  ll x, y;
  ll g = gcdExtended(b, m, &x, &y);
  if (g != 1)
    return -1;
  return (x % m + m) % m;
}

ll modDivide(ll a, ll b, ll m) {
  a = a % m;
  int inv = modInverse(b, m);
  return (inv * a) % m;
}

ll power(ll a, ll n) {
  ll ans = 1;
  while (n > 0) {
    ll lastbit = (n & 1);
    if (lastbit)
      ans *= a;

    a *= a;
    n >>= 1;
  }

  return ans;
}
/////////////////////////////////////////////////
int sieveo(ll n) {

  bool prime[n + 1];
  memset(prime, true, sizeof(prime));
  ll ans = 0;

  for (ll p = 2; p * p <= n; p++) {
    if (prime[p] == true) {

      for (ll i = p * p; i <= n; i += p)
        prime[i] = false;
    }
  }

  for (ll p = 2; p <= n; p++)
    if (prime[p])
      ans++;

  return ans;
}

///////////////////////////////

void primesieve(ll N, ll s[]) {
  vector<bool> prime(N + 1, false);

  for (ll i = 2; i <= N; i += 2)
    s[i] = 2;

  for (ll i = 3; i <= N; i += 2) {
    if (prime[i] == false) {
      s[i] = i;

      for (ll j = i; j * i <= N; j += 2) {
        if (prime[i * j] == false) {
          prime[i * j] = true;
          s[i * j] = i;
        }
      }
    }
  }
}

void primefactor(ll N) {

  ll s[N + 1];
  primesieve(N, s);

  ll curr = s[N];
  ll cnt = 1;

  while (N > 1) {
    N /= s[N];
    if (curr == s[N]) {
      cnt++;
      continue;
    }

    // curr==prime n.
    // cnt=power
    curr = s[N];
    cnt = 1;
  }
}

/////////////////////////////////////////////////////////

void solve() {
  ll n;
  cin >> n;
  ll sum = 0;
  for (ll i = 1; i <= n; ++i) {

    ll x = n / i;
    sum += (i * (x * (x + 1)) / 2);
  }

  cout << sum;
}

int main() {
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  // TEST
  solve();
}