Project Euler Factorial Digit Sum

Factorial

The factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n. For example, 4! = 4 * 3 * 2 * 1.

• Wikipedia - Factorial

Instructions

• Write a method, factorial, that returns the factorial of the number passed in. It should work for arbitrarily large n (at least up to n = 100).

  Example

  factorial(4) would return 24:

  • 4 * 3 -> 12
  • 4 * 3 * 2 -> 24
  • 4 * 3 * 2 * 1 -> 24

• Once the first test passes, write a method sum_of_digits that returns the sum of the digits of the number passed in: sum_of_digits(16) => 7.

• Finally, using the first two methods you wrote as helpers, define a factorial_digit_sum method and make the last test pass.

Source

• Project Euler - Problem 16

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