筛质数

（朴素筛法）将每个质数的倍数全部筛掉

下面是完整的代码：

#include <iostream>
#include <algorithm>

using namespace std;

const int N = 1000010;
int primes[N];
bool st[N];
int n, cnt;

void get_primes(int x)
{
    for (int i = 2; i <= n; i ++ )
    {
        if (st[i]) continue;
        primes[cnt ++ ] = i;

        for (int j = i + i; j <= n; j += i)
        {
            st[j] = true;
        }
    }
}

int main()
{
    cin >> n;

    get_primes(n);

    cout << cnt;

    return 0;
}