elephant.rb

f_lines = File.read("input.txt").lines.map(&:chomp)
result_lines = f_lines.map do |line|
  # Extract all angle brackets, and sort them.
  # We use this as a "stack" of angle brackets that we can pull from
  # when we need an angle bracket in the final result
  angle_brackets = line.scan(/[<>]/).sort

  line_chars = line.chars.map do |c|
    c == " " ? " " : angle_brackets.shift
  end
  line_chars.join
end

puts result_lines.join("\n")