874. 筛法求欧拉函数

Posted 2021-06-14 幽殇默

上图摘自Acwing的某位大佬：https://www.acwing.com/solution/content/28507/

https://www.acwing.com/problem/content/876/

#include<cstdio>
#include<iostream>
using namespace std;
const int N=1e6+10;
typedef long long int LL;
LL phi[N];
LL prime[N],cnt;
bool st[N];
LL get_prime(int n)
{
    phi[1]=1;
	for(int i=2;i<=n;i++)
	{
		if(!st[i])
		{
			prime[cnt++]=i;
			phi[i]=i-1;
		}
		for(int j=0;prime[j]<=n/i;j++)
		{
			st[prime[j]*i]=true;
			if(i%prime[j]==0)
			{
				phi[prime[j]*i]=phi[i]*prime[j];
				break;
			}
			phi[i*prime[j]]=phi[i]*(prime[j]-1);
		}
	}
	LL res=0;
	for(int i=1;i<=n;i++) res+=phi[i];
	return res;
}
int n;
int main(void)
{
	cin>>n;
	cout<<get_prime(n)<<endl;
	return 0;
}