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  1. #include <bits/stdc++.h>
  2. #define ll long long
  3.  
  4. using namespace std;
  5. const int N = 1e6 + 7;
  6.  
  7. int n;
  8. int a[N];
  9. bool prime[N];
  10.  
  11. void sieve(){
  12.     memset(prime, true, sizeof(prime));
  13.     prime[0] = prime[1] = 0;
  14.     for (int i = 2; i * i < N; i++) {
  15.         if (prime[i]) {
  16.             for (int j = i * i; j < N; j += i) {
  17.                 prime[j] = false;
  18.             }
  19.         }
  20.     }
  21. }
  22.  
  23. void sub1() {
  24.     int ans = 0;
  25.     for (int i = 1; i <= n; i++) {
  26.         for (int j = 1; j <= i; j++) {
  27.             if (i % j == 0 && j != i / j && prime[j] && prime[i / j]) {
  28.                 ans++;  
  29.                 break;
  30.             }
  31.         }
  32.     }
  33.     cout << ans << '\n';
  34. }
  35.  
  36. void sub2() {
  37.     int cnt = 0, ans = 0;
  38.     for (int i = 1; i < N; i++) if (prime[i]) a[++cnt] = i;
  39.     for (int i = 1; i <= cnt && a[i] <= n; i++) {
  40.         for (int j = i + 1; j <= cnt && 1ll * a[i] * a[j] <= n; j++) {
  41.             ans++;
  42.         }
  43.     }
  44.     cout << ans << '\n';
  45. }
  46.  
  47. signed main() {
  48.     ios_base::sync_with_stdio(0);
  49.     cin.tie(0); cout.tie(0);
  50.  
  51.     freopen("TICHSO.inp", "r", stdin);
  52.     freopen("TICHSO.out", "w", stdout);
  53.  
  54.     cin >> n;
  55.     sieve();
  56.     if (n <= 1000) sub1();
  57.     else sub2();
  58.     return 0;
  59. }
Success #stdin #stdout 0.01s 6444KB
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https://ideone.com/zsvbxM
language:
C++ (gcc 8.3)
created:
3 days ago
visibility:
public

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