solve31.rb

# https://projecteuler.net/problem=31
# Run with: 'ruby solve31.rb'
# using Ruby 2.5.1
# by Zack Sargent

# Prompt:

# In the United Kingdom the currency is made up of pound (£) and pence (p). There are eight coins in general circulation:
# 1p, 2p, 5p, 10p, 20p, 50p, £1 (100p), and £2 (200p).
# It is possible to make £2 in the following way:
# 1×£1 + 1×50p + 2×20p + 1×5p + 1×2p + 3×1p
# How many different ways can £2 be made using any number of coins?

COINS = [1, 2, 5, 10, 20, 50, 100, 200]

def get_ways(target) 
  arr = [1]
  COINS.each do |coin|
    (coin..target).each do |i|
      if arr[i]
        arr[i] += arr[i - coin]
      else 
        arr[i] = arr[i - coin]
      end
    end
  end
  arr.last
end

p get_ways(200)
# -> 73682