General solution to the Number Shuffle problem

number_shuffle.rb

# from https://rubymonk.com/learning/books/1-ruby-primer/problems/154-permutations
# Problem statement: Given a 3 or 4 digit number with distinct digits, return a sorted array of all the unique numbers that can be formed with those digits. 
# This solution will work for any number (within limits of memory and stack depth)

def number_shuffle(number)
  digits = number.to_s.split('')
  if digits.length == 1
    return [number]
  end
  combinations = []
  digits.each do |digit|
    remaining = digits.select { |d| d != digit }.join
    if remaining.length != 0
      sub_combinations = number_shuffle(remaining.to_i)
      sub_combinations.each do |sub_combination|
        combination = (digit.to_s + sub_combination.to_s).to_i
        combinations = combinations.push(combination)
      end
    end
  end
  return combinations
end