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P3912 素数个数
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problem/P3912/P3912.cpp
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23
problem/P3912/P3912.cpp
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#include <bits/stdc++.h>
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using namespace std;
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int main() {
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int n;
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cin >> n;
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int s = n-1;
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bool prime[100000005];
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memset(prime, 0, sizeof(prime));
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for(int i = 2 ; i*i <= n ; i++) {
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if(!prime[i]) {
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for(int j = i*2 ; j <= n ; j+= i) {
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if(!prime[j]) {
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prime[j] = true;
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s--;
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}
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}
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}
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}
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cout << s << endl;
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return 0;
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}
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