https://leetcode.com/problems/decode-string/submissions/

leetcode_394_decode_string.rb

# @param {String} s
# @return {String}
def decode_string(s)
  puts "decoding #{s.inspect}"
  digits = ('0'..'9').to_set
  result = ""
  
  multiplier = ""
  group = ""
  
  inside_group = false
  plain_text_group = true
  group_bracket_level = 0
  
  s.each_char do |char|  
    if digits.include?(char)
      if inside_group
        group << char
      else
        result << group
        group = ""
        multiplier << char 
      end
    elsif char == '['
      if inside_group
        group_bracket_level += 1
        plain_text_group = false
        group << char
      else
        inside_group = true
      end
    elsif char == ']'
      if group_bracket_level == 0
        integer_multiplier = multiplier == "" ? 1 : multiplier.to_i
        addition = if plain_text_group
          group * integer_multiplier
        else
          decode_string(group) * integer_multiplier
        end
        result << addition
        inside_group = false
        plain_text_group = true
        multiplier = ""
        group = ""
      else
        group_bracket_level -= 1
        group << char
      end
    else
      group << char
    end
  end

  integer_multiplier = multiplier == "" ? 1 : multiplier.to_i
  addition = if plain_text_group
    group * integer_multiplier
  else
    decode_string(group) * integer_multiplier
  end
  result << addition
  result
end