bzoj 3643 Phiの逆関数検索

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#define ll long long
using namespace std;

int n,m,cnt,c[100005]; bool vis[100005]; ll ans;
bool prm(int x){
	int sz=(int)sqrt(x),i;
	for (i=1; c[i]<=sz; i++) if (!(x%c[i])) return 0;
	return 1;
}
void dfs(int k,int now,ll sum){
	if (sum>=ans) return; int i;
	if (now==1){ ans=sum; return; }
	if (now>m && prm(now+1)) ans=min(ans,sum*(now+1));
	for (i=k+1; c[i]-1<=m; i++){
		if (c[i]-1>now) break;
		if (!(now%(c[i]-1))){
			int x=now/(c[i]-1); ll y=sum*c[i]; dfs(i,x,y);
			while (!(x%c[i])){
				x/=c[i]; y*=c[i];
				dfs(i,x,y);
			}
		}
	}
}
int main(){
	scanf("%d",&n); m=(int)sqrt(n); int i,j;
	for (i=2; i<=50000; i++){
		if (!vis[i]) c[++cnt]=i;
		for (j=1; j<=cnt && i*c[j]<=50000; j++){
			vis[i*c[j]]=1;
			if (!(i%c[j])) break;
		}
	}
	ans=(ll)1073741824*2; dfs(1,n,1);
	if (ans<=2147483647) printf("%lld
",ans);
		else puts("-1");
	return 0;
}