package problem20;

/*

Factorial Digit Sum

Problem 20

n! means n × (n − 1) × ... × 3 × 2 × 1

For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,

and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.

Find the sum of the digits in the number 100!

*/

import java.math.BigInteger;

public class FactorialDigitSum {

public static void main(String[] args) {

BigInteger answer = factorial(BigInteger.valueOf(100));

String digits = answer.toString();

int sum = 0;

for (int i = 0; i < digits.length(); i++) {

sum += Integer.parseInt("" + digits.charAt(i));

}

System.out.println(sum);

}

private static BigInteger factorial(BigInteger n) {

if (n.compareTo(BigInteger.valueOf(1)) == 0) {

return n;

}

return n.multiply(factorial(n.subtract(BigInteger.valueOf(1))));

}

}