WeChall writeup ：Prime Factory

题目：Prime Factory

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

解题：

找到两个质数，满足：

1.大于1000000

2.各位数字合也是质数。如23是质数，2+3=5也是质数。

写一段C程序，solution：10000331000037

    #include <stdio.h>      #include <stdlib.h>      #include <string.h>      #include <math.h>    int isprime1(int num){for(int m=2;m<=sqrt(num);m++){if(num%m==0)return 0; //num不是质数}return 1;//num是质数}int isprime2(int num){int sum=0;while(num){sum+=num%10;num/=10;}if(isprime1(sum))return 1; //各位数字之和仍为质数elsereturn 0;}          int main()      {          int i;          char temp[100]= "";          FILE*fp=fopen("prime1.txt","w");                             for(i=3;i<1000100;i+=2)          {  if(i>1000000){            if(isprime1(i))if(isprime2(i)){sprintf(temp, "%d\n", i);fwrite(temp,1,strlen(temp),fp); }}        }          fclose(fp);          printf("DONE!!\n");          return 0;      }