Project Euler Problem 20 Solution: Factorial digit sum

Project Euler Problem 20 Statement

$n!$ means $n\times (n-1)\times \dots \times 3\times 2\times 1$

Find the digit sum in the number $100!$

Solution

Python natively supports arbitrary-precision integers and arithmetic with as many digits as necessary to perform a calculation. For example, it takes 158 digits to represent 100! and Python handles it easily.

sum(map(int,str(math.factorial(100))))

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To solve this problem, we calculate 100! using the factorial function from the math library and convert the result to a string so we can iterate over each digit individually. Since integers are not directly iterable, we convert the result to a string, allowing us to access each digit individually.

Then, using the map() function, we convert every character-digit back to an integer and add them up for a digit sum using the sum() function.

HackerRank version

Solves all test cases for Hackerrank Project Euler Problem 20

HackerRank Project Euler 20 increases the factorial to any number $0\le N\le1000$ and runs 100 test cases.

Python Source Code

import math
for _ in range(int(input())):
    N = int(input())
    print(sum(map(int, str(math.factorial(N)))))

Last Word

Embrace the power of arbitrary-precision integers and ignore the call to solve it in some primitive way.