【BZOJ 3643】Phi的反函数 数搜索

#include <cstdio>
#include <cmath>
typedef long long LL;
int n;
const int N=100010;
const LL Inf=0x7fffffff;
LL ans;
int len,prime
;
bool isnot
;
inline void getprime(){
int lim=100000;
for(int i=2;i<=lim;i++){
if(!isnot[i])prime[++len]=i;
for(int j=1;prime[j]*i<=lim;j++){
isnot[prime[j]*i]=1;
if(i%prime[j]==0)break;
}
}
}
inline bool isprime(int x){
if(x==1)return 0;
int lim=(int)sqrt(x+0.5);
for(int i=1;prime[i]<=lim;i++)
if(x%prime[i]==0)
return 0;
return 1;
}
inline LL Min(LL x,LL y){
return x<y?x:y;
}
void dfs(int pos,LL now,int rest){
if(pos>=len)return;
if(now>=ans)return;
if(rest==1){ans=now;return;}
if(isprime(rest+1)){ans=Min(ans,now*(rest+1));return;}
LL have=prime[pos]-1,j=prime[pos];
if(pos==1)have<<=1,j<<=1;
while(rest%have==0)
dfs(pos+1,now*j,rest/have),have*=prime[pos],j*=prime[pos];
dfs(pos+1,now,rest);
}
int main(){
getprime(),scanf("%d",&n),ans=Inf+1;
if(n<=0){printf("-1");return 0;}
if(n==1){printf("1");return 0;}
dfs(1,1,n);
printf("%lld",ans==Inf+1?-1:ans);
}