bzoj 4173 打表???

没有任何思路，打表发现ans=phi(n)*phi(m)*n*m
%%% popoqqq Orz
然而并没有看懂……

#include<cstdio>#include<cstring>#include<iostream>#include<algorithm>#include<cmath>#define mod 998244353llusing namespace std;long long getphi(long long x){    long long ans=x;    for(long long i=2;i*i<=x;i++)        if(x%i==0){            ans/=i;ans*=i-1;            while(x%i==0)x/=i;        }    if(x!=1){ans/=x;ans*=x-1;}    return ans;}int main(){    long long  n,m,x,y,ans=1;    scanf("%lld%lld",&n,&m);    x=getphi(n)%mod;    y=getphi(m)%mod;    n%=mod; m%=mod;    ans=n;ans=(ans*m)%mod;    ans=(ans*x)%mod;ans=(ans*y)%mod;    printf("%lld\n",ans);    return 0;}

附打表程序

#include<cstdio>#include<cstring>#include<iostream>#include<algorithm>#include<cmath>#define N 500using namespace std;int phi[N],prime[N],tot;bool bo[N];void init(){    phi[1]=1;    for(int i=2;i<=N;i++){        if(!bo[i]){            prime[++tot]=i;            phi[i]=i-1;        }        for(int j=1;j<=tot&&i*prime[j]<=N;j++){            bo[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]];        }    }}void work(int x,int y){    int cnt=0;    printf("phi[%d]==%d  phi[%d]==%d\n",x,phi[x],y,phi[y]);    for(int i=2;i<=x+y;i++)        if(x%i+y%i>=i){            printf("phi[%d]==%d\n",i,phi[i]);            cnt+=phi[i];        }    printf("%d  %d  %d  %d  ans==%d\n",x,y,phi[x],phi[y],phi[x]*phi[y]*cnt);}int main(){    int x,y;    init();    while(scanf("%d%d",&x,&y)==2)        work(x,y);}