线性筛欧拉函数

#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);         }     } }

原文：https://www.cnblogs.com/water-radish/p/9280649.html