倒序算法

arr = [1,3,2,5,7,8,2,9]

for i in (0...arr.length)

  for j in ((i+1)...arr.length)    

    if arr[i] < arr[j] then

      temp = arr[i]

      arr[i] = arr[j]

      arr[j] = temp

    end

  end

end

puts arr