Problem p02624 (C++)

Buggy submission (status: Runtime Error):
/*
    @uthor: Amit Kumar
    user -->GitHub: drviruses ; CodeChef: dr_virus_ ;
   Codeforces,AtCoder,Hackerearth,Hakerrank: dr_virus;
*/
#include <bits/stdc++.h>
#include <chrono>
using namespace std;
using namespace std::chrono;
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision;
// #define ln mp::cpp_int;
#define int long long
#define ll long long
#define ld long double
#define ull unsigned long long
#define endl "\n"
ll google_itr = 1;
#define google(x) cout << "Case #" << x << ":"
#define pi 3.14159265358979323846264338327950L

const ll mod = 1e9 + 7;
const ll inf = 1e18;
const ll mx = 1e7 + 2;
ll dp[mx];
ll popcount(ll num) {
  num = num - ((num >> 1) & 0x55555555);
  num = (num & 0x33333333) + ((num >> 2) & 0x33333333);
  return ((num + (num >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
}

void SieveOfEratosthenes(int n, bool prime[], bool primesquare[], int a[]) {
  // Create a boolean array "prime[0..n]" and
  // initialize all entries it as true. A value
  // in prime[i] will finally be false if i is
  // Not a prime, else true.
  for (int i = 2; i <= n; i++)
    prime[i] = true;

  // Create a boolean array "primesquare[0..n*n+1]"
  // and initialize all entries it as false. A value
  // in squareprime[i] will finally be true if i is
  // square of prime, else false.
  for (int i = 0; i <= (n * n + 1); i++)
    primesquare[i] = false;

  // 1 is not a prime number
  prime[1] = false;

  for (int p = 2; p * p <= n; p++) {
    // If prime[p] is not changed, then
    // it is a prime
    if (prime[p] == true) {
      // Update all multiples of p
      for (int i = p * 2; i <= n; i += p)
        prime[i] = false;
    }
  }

  int j = 0;
  for (int p = 2; p <= n; p++) {
    if (prime[p]) {
      // Storing primes in an array
      a[j] = p;

      // Update value in primesquare[p*p],
      // if p is prime.
      primesquare[p * p] = true;
      j++;
    }
  }
}

// Function to count divisors
int fac(int n) {
  // If number is 1, then it will have only 1
  // as a factor. So, total factors will be 1.
  if (n == 1)
    return 1;

  bool prime[n + 1], primesquare[n * n + 1];

  int a[n]; // for storing primes upto n

  // Calling SieveOfEratosthenes to store prime
  // factors of n and to store square of prime
  // factors of n
  SieveOfEratosthenes(n, prime, primesquare, a);

  // ans will contain total number of distinct
  // divisors
  int ans = 1;

  // Loop for counting factors of n
  for (int i = 0;; i++) {
    // a[i] is not less than cube root n
    if (a[i] * a[i] * a[i] > n)
      break;

    // Calculating power of a[i] in n.
    int cnt = 1;          // cnt is power of prime a[i] in n.
    while (n % a[i] == 0) // if a[i] is a factor of n
    {
      n = n / a[i];
      cnt = cnt + 1; // incrementing power
    }

    // Calculating number of divisors
    // If n = a^p * b^q then total divisors of n
    // are (p+1)*(q+1)
    ans = ans * cnt;
  }

  // if a[i] is greater than cube root of n

  // First case
  if (prime[n])
    ans = ans * 2;

  // Second case
  else if (primesquare[n])
    ans = ans * 3;

  // Third casse
  else if (n != 1)
    ans = ans * 4;

  return ans; // Total divisors
}
void pre() {
  for (auto i = 0; i < mx + 1; i++) {
    dp[i] = 1;
  }
  for (auto i = 2; i <= mx; i++) {
    for (auto j = i; j <= mx; j++) {
      dp[j]++;
    }
  }
}

void virus() {
  pre();
  ll num;
  cin >> num;
  ll sum = 0;
  for (auto i = 1; i <= num; i++) {
    sum += (i * dp[i]);
  }
  cout << sum;
}

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);

  /*#ifndef ONLINE_JUDGE
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
    #endif*/

  ll t = 1;
  // cin>>t;
  while (t--) {
    auto start = high_resolution_clock::now();
    virus();
    auto stop = high_resolution_clock::now();
    auto duration = duration_cast<seconds>(stop - start);
    // cout << "Time: "<<duration.count()<<endl;
    // your code goes here
  }
  return 0;
}

Fixed submission:
/*
    @uthor: Amit Kumar
    user -->GitHub: drviruses ; CodeChef: dr_virus_ ;
   Codeforces,AtCoder,Hackerearth,Hakerrank: dr_virus;
*/
#include <bits/stdc++.h>
#include <chrono>
using namespace std;
using namespace std::chrono;
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision;
// #define ln mp::cpp_int;
#define int long long
#define ll long long
#define ld long double
#define ull unsigned long long
#define endl "\n"
ll google_itr = 1;
#define google(x) cout << "Case #" << x << ":"
#define pi 3.14159265358979323846264338327950L

const ll mod = 1e9 + 7;
const ll inf = 1e18;
const ll mx = 1e7 + 2;
ll dp[mx];
ll popcount(ll num) {
  num = num - ((num >> 1) & 0x55555555);
  num = (num & 0x33333333) + ((num >> 2) & 0x33333333);
  return ((num + (num >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
}

void SieveOfEratosthenes(int n, bool prime[], bool primesquare[], int a[]) {
  // Create a boolean array "prime[0..n]" and
  // initialize all entries it as true. A value
  // in prime[i] will finally be false if i is
  // Not a prime, else true.
  for (int i = 2; i <= n; i++)
    prime[i] = true;

  // Create a boolean array "primesquare[0..n*n+1]"
  // and initialize all entries it as false. A value
  // in squareprime[i] will finally be true if i is
  // square of prime, else false.
  for (int i = 0; i <= (n * n + 1); i++)
    primesquare[i] = false;

  // 1 is not a prime number
  prime[1] = false;

  for (int p = 2; p * p <= n; p++) {
    // If prime[p] is not changed, then
    // it is a prime
    if (prime[p] == true) {
      // Update all multiples of p
      for (int i = p * 2; i <= n; i += p)
        prime[i] = false;
    }
  }

  int j = 0;
  for (int p = 2; p <= n; p++) {
    if (prime[p]) {
      // Storing primes in an array
      a[j] = p;

      // Update value in primesquare[p*p],
      // if p is prime.
      primesquare[p * p] = true;
      j++;
    }
  }
}

// Function to count divisors
int fac(int n) {
  // If number is 1, then it will have only 1
  // as a factor. So, total factors will be 1.
  if (n == 1)
    return 1;

  bool prime[n + 1], primesquare[n * n + 1];

  int a[n]; // for storing primes upto n

  // Calling SieveOfEratosthenes to store prime
  // factors of n and to store square of prime
  // factors of n
  SieveOfEratosthenes(n, prime, primesquare, a);

  // ans will contain total number of distinct
  // divisors
  int ans = 1;

  // Loop for counting factors of n
  for (int i = 0;; i++) {
    // a[i] is not less than cube root n
    if (a[i] * a[i] * a[i] > n)
      break;

    // Calculating power of a[i] in n.
    int cnt = 1;          // cnt is power of prime a[i] in n.
    while (n % a[i] == 0) // if a[i] is a factor of n
    {
      n = n / a[i];
      cnt = cnt + 1; // incrementing power
    }

    // Calculating number of divisors
    // If n = a^p * b^q then total divisors of n
    // are (p+1)*(q+1)
    ans = ans * cnt;
  }

  // if a[i] is greater than cube root of n

  // First case
  if (prime[n])
    ans = ans * 2;

  // Second case
  else if (primesquare[n])
    ans = ans * 3;

  // Third casse
  else if (n != 1)
    ans = ans * 4;

  return ans; // Total divisors
}
void pre() {
  for (auto i = 0; i < mx + 1; i++) {
    dp[i] = 1;
  }
  for (auto i = 2; i <= mx; i++) {
    for (auto j = i; j <= mx; j += i) {
      dp[j]++;
    }
  }
}

void virus() {
  pre();
  ll num;
  cin >> num;
  ll sum = 0;
  for (auto i = 1; i <= num; i++) {
    sum += (i * dp[i]);
  }
  cout << sum;
}

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);

  /*#ifndef ONLINE_JUDGE
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
    #endif*/

  ll t = 1;
  // cin>>t;
  while (t--) {
    auto start = high_resolution_clock::now();
    virus();
    auto stop = high_resolution_clock::now();
    auto duration = duration_cast<seconds>(stop - start);
    // cout << "Time: "<<duration.count()<<endl;
    // your code goes here
  }
  return 0;
}