def tokenize(code)
  tokens = []
  code = code.dup

  while !code.empty?
    if ['(', ')'].include? code[0,1]
      tokens << code[0,1]
      code = code[1...code.size]
    elsif code[0,1] == ' '
      code = code[1...code.size]
    else
      m = /[a-zA-Z0-9+-=*\/?#]+/.match(code)
      tokens << m[0]
      code = code[m[0].size...code.size]
    end
  end
  tokens
end

def parse(tokens)
  input = tokens.dup
  stack = []
  current = stack
  previous = []

  while !input.empty?
    if input.first == '('
      previous << current
      current << []
      current = current.last
      input = input[1...input.size]
    elsif input.first == ')'
      current = previous.pop
      input = input[1...input.size]
    else
      current << input.first
      input = input[1...input.size]
    end
  end
  stack.first
end

p parse(tokenize("(def (f x) (h 23 ((g x) 24)))"))
p parse(tokenize("(define (fact n) (if (= n 0) 1 (* n (fact (- n 1)))))"))