class MergeSort

  def self.merge_sort(list)
    return list if list.length <= 1
    mid = list.length / 2
    left = merge_sort(list[0...mid])      # a...b is from a to b exluding b
    right = merge_sort(list[mid..-1])
    merge(left, right)
  end

  def self.merge(left, right)
    arr = []
    i = j = 0
    while i < left.length and j < right.length do
      if left[i] <= right[j]
        arr << left[i]        # no need for a counter for arr, because Array#<< or Array#push will simply append the object
        i += 1
      else
        arr << right[j]
        j += 1
      end
    end
    arr.concat left[i..-1]    # no need to check for i < left.length, because left[i..-1] will simply be empty in this case
    arr.concat right[j..-1]
    arr
  end

end

list = [7,6,5,4,3,2,1]
p MergeSort.merge_sort(list)