Project Euler #1 - are we fizz-buzzing?

#kotlin #projecteuler #beginners #programming

Disclaimer

This blogpost provides solution to the first problem from Project Euler. If
you want to figure out a solution yourself, go to the
Problem 1's page and come back later :)

This is going to be a very brief post as the problem is very simple as well.
It's similar to pretty well known FizzBuzz problem, which is a basic
recruiting task or a simple task at the beginning of one's programming-learning journey.

The Problem

If we list all the natural numbers below 10 that are multiples of 3 or 5,
we get 3, 5, 6 and 9. The sum of these multiples is 23. Find the sum of all the multiples of 3 or 5 below 1000.

The simplest O(n) solution would be iteration over the numbers and find multiples of 3 and 5.

Solution

The simplest solution using a loop would like something like this:

private fun loopApproach(input: Int): Int {
    var acc = 0
    for (i in 3 until input) {
        if (i.mod(3) == 0 || i.mod(5) == 0) {
            acc += i
        }
    }

    return acc
}

To be more kotlin you could use fold instead of usual for-loop:

private fun foldApproach(input: Int): Int =
    (3 until input).fold(0) { acc, value ->
        if (value.mod(3) == 0 || value.mod(5) == 0) {
            acc + value
        } else {
            acc
        }
    }

The fold, however, is a bit slower. With large input value
(1_000_000_000 instead of 1_000 and I need use Long for that) which takes in both cases almost 3 seconds on my computer, the difference is over
100ms.