Solution to Project Euler Problem 20: Factorial digit sum - 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!

• Java Code

package sikademy;
/**
 *
 * @author Archangel Macsika
 * Copyright (c) Sikademy. All rights reserved
 */

import java.math.BigInteger;

public class SikademyEulerSolution {
	
	public String run() {
		String temp = factorial(100).toString();
		int sum = 0;
		for (int i = 0; i < temp.length(); i++)
			sum += temp.charAt(i) - '0';
		return Integer.toString(sum);
	}

        // Returns n!.
	public static BigInteger factorial(int n) {
		if (n < 0)
			throw new IllegalArgumentException("Factorial of negative number");
		BigInteger prod = BigInteger.ONE;
		for (int i = 2; i <= n; i++)
			prod = prod.multiply(BigInteger.valueOf(i));
		return prod;
	}

	public static void main(String[] args) {
                SikademyEulerSolution solution = new SikademyEulerSolution();
		System.out.println(solution.run());
	}	
}

• Python Code

# 
# @author Archangel Macsika
# Copyright (c) Sikademy. All rights reserved.
# 

import math

# We do a straightforward computation thanks to Python's built-in arbitrary precision integer type.
def compute():
	n = math.factorial(100)
	ans = sum(int(c) for c in str(n))
	return str(ans)


if __name__ == "__main__":
	print(compute())