twenty_four.rb

def twenty_four(*operands)
  p operands
  operators = %w(+ - * /)
  operands.map! {|op| op.to_f}

  operands.permutation.to_a.uniq.each do |ordered_operands|
    operators.repeated_permutation(operands.length-1) do |ordered_operators|
      check_answer(ordered_operands, ordered_operators)
    end
  end

  puts ''
end


def check_answer(operands, operators)
  add_parentheses(operands).each do |p_operands|
    formula = make_formula(p_operands, operators)
    puts "#{make_formula(p_operands.map {|op| op.is_a?(Numeric) ? op.to_i : op}, operators)}=24" if eval(formula) == 24
  end
end


def add_parentheses(sequence)
  results = []
  if sequence.length <= 1
    results << sequence
  else
    0.upto(sequence.length-2) do |i|
      add_parentheses(sequence[0..i]).each do |left|
        add_parentheses(sequence[(i+1)..-1]).each do |right|
          results << ['('] + left + right + [')']
        end
      end
    end
  end
  results
end


def make_formula(p_operands, operators)
  formula = ''
  this_operators = operators.clone
  p_operands.each_with_index do |operand, i|
    formula += operand.to_s
    formula += this_operators.shift if operand.is_a?(Numeric) && p_operands[i+1] != ')'
    formula += this_operators.shift if operand  == ')' && p_operands[i+1].is_a?(Numeric)
    formula += this_operators.shift if operand  == ')' && p_operands[i+1] == '('
  end
  formula
end

twenty_four(2, 3, 4)
twenty_four(5, 1, 5)
twenty_four(3, 8)
twenty_four(3, 4, 5, 6)