Here is my solution:

http://pastie.caboo.se/124288
I'm pretty it removes every unnecessary (), at least for - + ** ^ /.

Thanks for the quiz!

On 02/12/2007, Robert Dober <robert.dober / gmail.com> wrote:
> Here goes my solution for this Quiz, I am confident that solutions 1
> and 2 are correct, well 3 too, for 4 that is less sure ;).
> Nice to have a quiz which can be solved in less time again.
>
> Robert
> ###############################
> # Read postfix from args or stdin
> # Print an infix solution *without* paranthesis
> postfix = ARGV.empty? ? \$stdin.read.split : ARGV
> postfix = postfix.map{ | ele | Integer( ele ) rescue ele }
> stack = []
> postfix.each do | ele |
>   case ele
>     when Integer
>       stack << ele
>     else
>       rhs = stack.pop
>       stack[ -1 ] = stack[ -1 ].send( ele, rhs )
>     end
> end
> puts stack.first.to_s
> #########################################################"
> # Read postfix from args or stdin
> # Print an infix solution with *many* paranthesis
>
> postfix = ARGV.empty? ? \$stdin.read.split : ARGV
> postfix = postfix.map{ | ele | Integer( ele ) rescue ele }
> stack = []
> postfix.each do | ele |
>   case ele
>     when Integer
>       stack << ele
>     else
>       rhs = stack.pop
>       stack[ -1 ] = "( #{stack[ -1 ]} ) #{ele}  ( #{rhs} )"
>     end
> end
>
> puts stack.first
> ##################################################
> # Read postfix from args or stdin
> # Print an infix solution with *some* paranthesis
> # the stupid ( and expensive ) way.
>
> class Expression
>   Combinations = [
>     ["", "", "", ""],
>     ["( ", " )", "", ""],
>     ["", "", "( ", " )"],
>     ["( ", " )", "( ", " )"]
>   ]
>   def initialize text
>     @value = Integer( text )
>     @text = text
>   end
>   def apply op, rhs
>     new_text = "#@text #{op} #{rhs.text}"
>     @value = @value.send( op, rhs.value )
>     Combinations.each do | parens |
>       txt = ["", @text, " #{op} ", rhs.text ].
>         zip( parens ).flatten.join
>         if eval( txt ) == @value then
>           return @text = txt
>         end
>     end
>     raise RuntimeError, "ooops"
>   end
> end
>
> postfix = ARGV.empty? ? \$stdin.read.split : ARGV
> postfix = postfix.map{ | ele | Expression.new( ele ) rescue ele }
> stack = []
> postfix.each do | ele |
>   case ele
>     when Expression
>       stack << ele
>     else
>       rhs = stack.pop
>       stack.last.apply ele, rhs
>     end
> end
>
> puts stack.first.text
> #############################################
> # Read postfix from args or stdin
> # Print an infix solution with *some* paranthesis
> # (the clever way?)
>
> class Expression
>   Priorities = {
>     "**" => 2,
>     "*" => 1, "/" => 1, "%" => 1,
>     "+" => 0, "-" => 0,
>     nil => 3
>   }
>   Commutative = %w[ * + ]
>   def initialize text
>     @top_op = nil
>     @text = text
>   end
>
>   def apply op, rhs
>     @text = parented_text( op ) +
>       " #{op} " << rhs.parented_text( op, false )
>     @top_op = op
>   end
>
>   def comm? op
>     Commutative.include? op
>   end
>
>   def parented_text op, is_lhs=true
>     my_prio = Priorities[ @top_op ]
>     op_prio  = Priorities[ op ]
>     return @text if op_prio < my_prio
>     return "( #@text )" if op_prio > my_prio
>     return @text if comm?( op ) || is_lhs
>     "( #@text )"
>   end
>
> end
> postfix = ARGV.empty? ? \$stdin.read.split : ARGV
> postfix = postfix.map{ | ele |
>   Expression::Priorities[ ele ] ? ele : Expression.new( ele )
> }
> stack = []
> postfix.each do | ele |
>   case ele
>     when Expression
>       stack << ele
>     else
>       rhs = stack.pop
>       stack[ -1 ].apply ele, rhs
>     end
> end
>
> puts stack.first.text
>
>