merge_sort.rb

def merge_sort(arr)

  # if array is empty or has just one element, no need to sort
  return arr if arr.size <= 1

  # split array in 2 halves (divide)
  mid = arr.size/2
  left = arr[0, mid] # arr[start,length]
  right = arr[mid, arr.size-mid]
  return merge(merge_sort(left),merge_sort(right))

end

def merge(arr1,arr2)
  merged_arr = []
  until arr1.empty? || arr2.empty?
    if arr1.first < arr2.first
      merged_arr << arr1.shift # removes first element of arr1 and also returns it (and pushes it to merged_arr)
    else
      merged_arr << arr2.shift
    end
  end
  merged_arr.concat(arr1).concat(arr2)
end

puts "What is the array you want to sort? (e.g. 56,2,34,1,4,75)"
array = gets.chomp.split(",").map { |i| i.to_i}
puts "Original array: #{array}"
puts "Sorted array: #{merge_sort(array)}"