Problem p02548 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
using namespace std;

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

#define int long long int
#define ld long double
#define pb push_back
#define MOD 1000000007
#define inf 3e18
#define vi vector<int>
#define vld vector<ld>
#define pii pair<int, int>
#define mii map<int, int>
#define fi first
#define se second
#define fastIO                                                                 \\
  ios_base::sync_with_stdio(0);                                                \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
#define db(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1> void __f(const char *name, Arg1 &&arg1) {
  cerr << " " << name << " : " << arg1 << '\\n';
}
template <typename Arg1, typename... Args>
void __f(const char *names, Arg1 &&arg1, Args &&...args) {
  const char *comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}

typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    pbds;
// order_of_key (k) : Number of items strictly smaller than k .
// find_by_order(k) : K-th element in a set (counting from zero) (returns an
// iterator)

void inp_out() {
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
  freopen("debug.txt", "w", stderr);
#endif
}

int32_t main() {
  fastIO inp_out();
  int n;
  cin >> n;

  int ans = 0, x = 0;
  for (int a = 1; a < n; ++a)
    ans += (n - 1) / a;
  cout << ans;
  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

#define int long long int
#define ld long double
#define pb push_back
#define MOD 1000000007
#define inf 3e18
#define vi vector<int>
#define vld vector<ld>
#define pii pair<int, int>
#define mii map<int, int>
#define fi first
#define se second
#define fastIO                                                                 \\
  ios_base::sync_with_stdio(0);                                                \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
#define db(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1> void __f(const char *name, Arg1 &&arg1) {
  cerr << " " << name << " : " << arg1 << '\\n';
}
template <typename Arg1, typename... Args>
void __f(const char *names, Arg1 &&arg1, Args &&...args) {
  const char *comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}

typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    pbds;
// order_of_key (k) : Number of items strictly smaller than k .
// find_by_order(k) : K-th element in a set (counting from zero) (returns an
// iterator)

void inp_out() {
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
  freopen("debug.txt", "w", stderr);
#endif
}

int32_t main() {
  fastIO inp_out();
  int n;
  cin >> n;

  int ans = 0;
  for (int i = n - 1; i >= 1; --i) {
    for (int j = 1; j * j <= i; ++j) {
      if (i % j == 0) {
        ans += 2;
        if (j == i / j)
          --ans;
      }
    }
  }
  cout << ans;
  return 0;
}