bzoj4805: 欧拉函数求和

杜教筛 

网上讲解可多了，我也刚学会？好像是会了就不讲了

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<string>
#include<map>
using namespace std;
long long phi[160005];
map<int,long long>G;
int n;
void lcr()
{
int i,k,l,j,t;
long long p;
for(i=2;i<=16000;i++){
k=0;t=i;p=i;
for(j=2;j<=sqrt(t);j++){
if(t%j==0)p=(p-p/j);
while(t%j==0)t/=j;
}
if(t!=1)p=(p-p/t);
phi[i]=p;
phi[i]+=phi[i-1];
}
}
long long find(int x)
{
int i,r=x,l=2;
long long ans=0;
if(x<=16000)return phi[x];
if(G[x])return G[x];
ans=(long long)x*(x+1)/2;
while(l<=x){
r=x/(x/l);
if(x/l>16000)ans-=(long long)(r-l+1)*find(x/l);
else ans-=(long long)(r-l+1)*phi[x/l];
l=r+1;
}
return G[x]=ans;
}
int main()
{
freopen("a.in","r",stdin);
freopen("a.out","w",stdout);
int i,j,k,l;
scanf("%d",&n);
phi[1]=1;
lcr();
printf("%lld",find(n));
return 0;
}