We Chall-Prime Factory-Writeup

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We Chall-Prime Factory-Writeup

题目链接：http://www.wechall.net/challenge/training/prime_factory/index.php

网站打开较慢，所以把题目放在下方：

Prime Factory (Training, Math)

Your task is simple:
Find the first two primes above 1 million, whose separate digit sums are also prime.
As example take 23, which is a prime whose digit sum, 5, is also prime.
The solution is the concatination of the two numbers,
Example: If the first number is 1,234,567
and the second is 8,765,432,
your solution is 12345678765432

大意就是找到两个数，这两个数符合：是大于1000000的质数，并且数字的每一位加起来仍是质数，如：23是质数，2+3=5仍是质数；

因为没有时间的限制，所以用C++写了一个暴力循环的代码：

 #include<iostream>
 #include<cmath>
 using namespace std;
 typedef long long LL;
 const LL MILLION = ;

 bool IsPrime_1(LL num)
 {
     for(int i = ; i <= sqrt(num); i++)
         if(!(num % i))
             return ;//num不是质数

     return ;//num是质数
 }

 bool IsPrime_2(LL num)
 {
     LL sum = ;
     while(num)
     {
         sum += num % ;
         num /= ;
     }

     if(IsPrime_1(sum))
         return ;//各位数字的和仍为质数
     else
         return ;
 }

 int main()
 {
 //    LL num;
 //    cin>>num;
 //    if(IsPrime_1(num))
 //        cout<<1<<endl;
 //    if(IsPrime_2(num))
 //        cout<<2<<endl;
     for(LL i = MILLION+ , cnt =  ; cnt <  ; i++)
     {
         if(IsPrime_1(i))
             if(IsPrime_2(i))
             {
                 cnt++;
                 cout<<i<<endl;
             }
     }

     return ;
 }

运行结果如下：

根据要求的形式得flag为:10000331000037

2017-2-5 12:40;11