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#### Project Euler 20 Problem Statement

n! means n × (n − 1) × … × 3 × 2 × 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.

The factorial function is imported from the Python math library.

`from math import factorial`

To solve this problem we calculate 100! and convert the result to a string so we can iterate over each character-digit.1

`sum(map(int, str(factorial(100))))`

Then, using the `map` function we iterate over the string and convert every character-digit back to an integer using the `int` function.

Finally, we add the digits together for a digit sum using the `sum` function.

`sum(map(int, str(factorial(100))))`

#### HackerRank version

HackerRank’s Project Euler 20 increases the factorial from 100 to 1,000 and runs 100 test cases.  This solution was fast enough to handle these parameters without change.

#### Last Word

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

1. integers are not iterable, but strings are.