线性筛欧拉函数

Posted 2020-12-02 water-radish

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<algorithm>
#define ll long long
using namespace std;
ll n,jdg[1000001],prime[1000001],phi[1000001],cnt,ans;
int main()
{
    scanf("%lld",&n);
    phi[1]=1;jdg[1]=1;
    for(ll i=2;i<=n;i++)
    {
        if(!jdg[i])
        {
            prime[++cnt]=i;
            phi[i]=i-1;
        }
        for(ll j=1;j<=cnt;j++)
        {
            if(i*prime[j]>n)
                break;
            jdg[i*prime[j]]=1;
            if(i%prime[j]==0)
            {
                phi[i*prime[j]]=phi[i]*prime[j];
                break;
            }
            phi[i*prime[j]]=phi[i]*(prime[j]-1);
        }
    }
}