868. 筛质数

Posted 2021-05-23 幽殇默

https://www.acwing.com/problem/content/870/

#include<cstdio>
#include<iostream>
using namespace std;
const int N=1e5+10;
int prime[N],cnt;
bool st[1000005];
int n;
void f(int x)
{
	for(int i=2;i<=x;i++)
	{
		if(!st[i]) prime[cnt++]=i;
		for(int j=0;prime[j]<=x/i;j++)
		{
			st[prime[j]*i] =true;
			if(i%prime[j]==0) break;
		}
	} 
} 
int main(void)
{
	cin>>n;
	f(n);
	cout<<cnt; 
	return 0;
}