Euler 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!

Write our own factorial implementation and get the solution.

In [1]:
def factorial(n):
    assert(n > 0)
    if n == 1:
        return 1
    else:
        return n * factorial(n - 1)
In [2]:
f100 = factorial(100)
print(sum(map(int, str(f100))))
assert(sum(map(int, str(f100))) == 648)

648