BZOJ2005 [Noi2010]能量采集

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
const int maxn=100000+10;

int not_prime[maxn],prime[maxn],phi[maxn],sumphi[maxn],pcnt;

inline void init_phi()
{
int n=100000;phi[1]=1;
for(int i=2;i<=n;++i)
{
if(!not_prime[i]) {
prime[++pcnt]=i;
phi[i]=i-1;
}

for(int j=1;j<=pcnt&&prime[j]*i<=n;++j)
{
not_prime[i*prime[j]]=1;
if(i%prime[j]==0){
phi[i*prime[j]]=phi[i]*prime[j];break;
}
else phi[i*prime[j]]=phi[i]*phi[prime[j]];
}
}

for(int i=1;i<=n;++i) sumphi[i]=sumphi[i-1]+phi[i];
}

int main()
{
init_phi();
int n,m;cin>>n>>m;
if(n>m) swap(n,m);

unsigned long long ans=0;
for(int d=1,j;d<=n;d=j+1)
{
j=min(n/(n/d),m/(m/d));
ans+=1ll*(n/d)*(m/d)*(sumphi[j]-sumphi[d-1]);
}
cout<<ans*2-1ll*n*m;
return 0;
}

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
typedef long long ll;
const int maxn=100000+10;

ll f[maxn];//f[i]表示i为gcd

int main()
{
int n,m;cin>>n>>m;
if(n>m) swap(n,m);
ll ans=0;
for(int i=n;i>=1;--i){
ll cnt=1ll*(n/i)*(m/i);
for(int j=i*2;j<=n;j+=i) cnt-=f[j];
f[i]=cnt;ans+=cnt*(i+i-1);
}
cout<<ans;
return 0;
}