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24 lines
543 B
24 lines
543 B
#include <bits/stdc++.h>
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using namespace std;
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const int N = 1e6 + 10;
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int primes[N], cnt; // primes[]存储所有素数
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int st[N]; // st[x]存储x是否被筛掉
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// 埃拉筛
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void get_primes(int n) {
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for (int i = 2; i <= n; i++)
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if (!st[i]) {
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primes[cnt++] = i; // 记录素数
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for (int j = 2 * i; j <= n; j += i)
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st[j] = 1; // 成倍数的标识
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}
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}
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int main() {
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int n;
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cin >> n;
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get_primes(n);
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printf("%d\n", cnt);
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return 0;
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} |