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868. 筛质数
https://www.acwing.com/problem/content/submission/code_detail/6873993/
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AcWing/868/868.cpp
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26
AcWing/868/868.cpp
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#include <bits/stdc++.h>
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using namespace std;
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bool isprime[1000005];
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int n, ans;
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int main() {
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cin >> n;
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for (int i = 0; i <= n; i++) {
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isprime[i] = true;
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}
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isprime[0] = isprime[1] = false;
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for (int i = 2; i <= n; i++) {
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if (isprime[i] && 1ll * i * i <= n) {
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for (int j = i * i; j <= n; j += i) {
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isprime[j] = false;
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}
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}
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}
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for (int i = 1; i <= n; i++) {
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ans += isprime[i];
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}
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cout << ans;
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return 0;
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}
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