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 = 1000000;

bool IsPrime_1(LL num)
{
    for(int i = 2; i <= sqrt(num); i++)
        if(!(num % i))
            return 0;//num不是质数

    return 1;//num是质数
}

bool IsPrime_2(LL num)
{
    LL sum = 0;
    while(num)
    {
        sum += num % 10;
        num /= 10;
    }

    if(IsPrime_1(sum))
        return 1;//各位数字的和仍为质数
    else
        return 0;
}

int main()
{
//    LL num;
//    cin>>num;
//    if(IsPrime_1(num))
//        cout<<1<<endl;
//    if(IsPrime_2(num))
//        cout<<2<<endl;
    for(LL i = MILLION+1 , cnt = 0 ; cnt < 2 ; i++)
    {
        if(IsPrime_1(i))
            if(IsPrime_2(i))
            {
                cnt++;
                cout<<i<<endl;
            }
    }

    return 0;
}

运行结果如下：

根据要求的形式得flag为:10000331000037

2017-2-5 12:40;11