UVA 10837 A Research Problem

https://vjudge.net/problem/UVA-10837

求最小的n，使phi（n）=m

#include<cstdio>
#include<algorithm>
#define N 10011
int prime[N],cnt;
bool v[N],vis[N];
int p[N],tot,ans;
void pre_prime()
{
    for(int i=2;i<N;i++)
    {
        if(!v[i]) prime[++cnt]=i;
        for(int j=1;j<=cnt;j++)
        {
            if(i*prime[j]>=N) break;
            v[i*prime[j]]=1;
            if(i%prime[j]==0) break;
        }
    }
}
int judge(int n)
{
    if(n==1) return 1;
    n++;
    for(int i=1;i<=cnt && prime[i]*prime[i]<=n;i++)
        if(n%prime[i]==0) return -1;
    for(int i=1;i<=tot;i++)
        if(vis[i] && n==p[i]) return -1;
    return n;
}
void dfs(int now,int left,int sum)
{
    if(sum==tot+1)
    {
        int ret=judge(left);
        if(ret>0) ans=std::min(ans,now*ret);
        return;
    }
    dfs(now,left,sum+1);
    if(left%(p[sum]-1)==0)
    {
        vis[sum]=1;
        now*=p[sum];
        left/=p[sum]-1;
        while(1)
        {
            dfs(now,left,sum+1);
            if(left%p[sum]) break;
            left/=p[sum];now*=p[sum];
        }
        vis[sum]=0;
    }
}
void solve(int n)
{
    tot=0; ans=2e9;
    for(int i=1;i<=cnt && n>=(prime[i]-1)*(prime[i]-1);i++)
        if(n%(prime[i]-1)==0)
        p[++tot]=prime[i];
    dfs(1,n,1);
}
int main()
{
    pre_prime();
    int n,t=0;
    while(scanf("%d",&n)!=EOF)
    {
        if(!n) return 0;
        solve(n);
        printf("Case %d: %d %d\n",++t,n,ans);
    }
}