题目链接
https://lydsy.com/JudgeOnline/problem.php?id=4805
题解
直接杜教筛筛函数即可。
代码
#include <map>
#include <cstdio>
#include <algorithm>
int read()
{
int x=0,f=1;
char ch=getchar();
while((ch<'0')||(ch>'9'))
{
if(ch=='-')
{
f=-f;
}
ch=getchar();
}
while((ch>='0')&&(ch<='9'))
{
x=x*10+ch-'0';
ch=getchar();
}
return x*f;
}
const int maxn=1587401;
int p[maxn+10],prime[maxn+10],cnt;
long long phi[maxn+10];
int getprime()
{
p[1]=phi[1]=1;
for(int i=2; i<=maxn; ++i)
{
if(!p[i])
{
prime[++cnt]=i;
phi[i]=i-1;
}
for(int j=1; (j<=cnt)&&(i*prime[j]<=maxn); ++j)
{
int x=i*prime[j];
p[x]=1;
if(i%prime[j]==0)
{
phi[x]=phi[i]*prime[j];
break;
}
phi[x]=phi[i]*(prime[j]-1);
}
}
for(int i=1; i<=maxn; ++i)
{
phi[i]+=phi[i-1];
}
return 0;
}
std::map<int,long long> mp;
long long getsum(int n)
{
if(n<=maxn)
{
return phi[n];
}
if(mp.count(n))
{
return mp[n];
}
long long ans=1ll*n*(n+1)/2;
for(int l=2,r; l<=n; l=r+1)
{
r=n/(n/l);
ans-=(r-l+1)*getsum(n/l);
}
return mp[n]=ans;
}
int n;
int main()
{
getprime();
n=read();
printf("%lld\n",getsum(n));
return 0;
}