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!

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Idea

As instructed, calculate factorial and sum all digits

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from functools import reduce
from operator import mul

def factorial(n):
    return reduce(mul, range(1, n+1), 1)

def solve(n):
    f = factorial(n)
    s = 0
    while f:
        s += f % 10
        f //= 10
    return s

solve(10)

27

solve(100)

648