[Project Euler] Problem 30

Surprisingly there are only three numbers that can be written as the sum of fourth powers of their digits:

1634 = 1 ⁴ + 6 ⁴ + 3 ⁴ + 4 ⁴
8208 = 8 ⁴ + 2 ⁴ + 0 ⁴ + 8 ⁴
9474 = 9 ⁴ + 4 ⁴ + 7 ⁴ + 4 ⁴

As 1 = 1⁴ is not a sum it is not included.

The sum of these numbers is 1634 + 8208 + 9474 = 19316.

Find the sum of all the numbers that can be written as the sum of fifth powers of their digits.

这道题没有太多技巧可言，直接穷举法

由于9**5*7 < 1000000；所以满足条件的数不可能是7位或以上的数

我们从2穷举到 9**5*6；

#include < iostream >
#include < cmath >
using namespace std;

bool isGood( int );

int main(){
int sum = 0 ;
for ( int i = 2 ; i < pow( 9 , 5 ) * 6 ; i ++ ){
if (isGood(i)){
sum += i;
}
}
cout << sum << endl;

}

bool isGood( int num){
int tmp = 0 ;
int backup = num;
while (backup > 0 ){
tmp += pow(backup % 10 , 5 );
backup /= 10 ;
}
return tmp == num;
}