记忆化搜索的方式计算f(x)

#include<cstdio>
#include<cmath>
#include<cstring>
#include<algorithm>
using namespace std;
#define mem(a) memset(a,0,sizeof(a))
const int maxn = 1000005;

int n;
int primes[maxn],prime_cnt;
int vis[maxn];
int v[maxn];
double f[maxn];

void prime_(){
    mem(v);
    for(int i=2;i<=(int)sqrt(maxn);i++){
        for(int j=i*i;j<=maxn;j+=i)if(!v[j]){
            v[j]=1;
        }
    }
    prime_cnt=0;
    for(int i=2;i<maxn-4;i++){
        if(!v[i])
            primes[prime_cnt++]=i;
    }
}
double dp(int x){
    if(x==1) return 0.0;
    if(vis[x]) return f[x];
    vis[x] = 1;
    double& ans = f[x];
    int g=0,p=0;
    ans=0;
    for(int i=0;i<prime_cnt&&primes[i] <= x;i++){
        p++;
        if(x%primes[i]==0){
            g++;
            ans+=dp(x/primes[i]);
        }
    }
    ans = (ans+p)/g;
    return ans;
}
int main()
{
//    freopen("out.txt","w",stdout);
    int t;
    scanf("%d\n",&t);
    mem(vis);mem(f);
    prime_();
    int kase=0;
    while(t--){
        scanf("%d",&n);
        double ans = dp(n);
        printf("Case %d: %.10lf\n",++kase,ans);
    }
    return 0;
}

uva 11762 Race to 1