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There are many different ways to write mathematical equations.  Infix notation
is probably the most popular and yields expressions like:

2 * (3 + 5)

Some people like to work with a postfix notation (often called Reverse Polish
Notation or just RPN) though, which doesn't require parentheses for the same
equation:

2 3 5 + *

You can compare the results of these equations using the Unix utilities bc
(infix) and dc (postfix):

\$ bc <<< '2 * (3 + 5)'
16
\$ dc <<< '2 3 5 + * p'
16

The "p" instruction tacked onto the end of the expression for dc just tells it
to print the result.

This week's quiz is to write a script that translates postfix expressions into
the equivalent infix expression.  In the simplest form, your script should
function as such:

\$ ruby postfix_to_infix.rb '2 3 +'
2 + 3

At minimum, try to support the four basic math operators:  +, -, *, and /.  Feel
free to add others though.  For numbers, remember to accept decimal values.

You can count on the postfix expressions having spaces between each term, if you
like.  While dc is content with 2 3+p, you don't have to support it unless you
want to.

For an added bonus, try to keep the parentheses added to infix expressions to
the minimum of what is needed.  For example, prefer these results:

\$ ruby postfix_to_infix.rb '56 34 213.7 + * 678 -'
56 * (34 + 213.7) - 678
\$ ruby postfix_to_infix.rb '1 56 35 + 16 9 - / +'
1 + (56 + 35) / (16 - 9)

to these:

\$ ruby postfix_to_infix.rb '56 34 213.7 + * 678 -'
((56 * (34 + 213.7)) - 678)
\$ ruby postfix_to_infix.rb '1 56 35 + 16 9 - / +'
(1 + ((56 + 35) / (16 - 9)))

Posting equations and your output is not a spoiler.

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