Reverse Polish Notation Calculator is a mathematical notation in which every operator follows all of its operands which puts the operator in the prefix position.
For example:
To add 3 and 4 we would normally write it "3 + 4". In reverse polish notation we would write this as "3 4 +"
If there are multiple operations, the operator is give immediately after its second operand; so "3 4 - 5 +" would be expressed as "3 - 4 + 5". Four is first subtracted from 3, then 5 added to it.
We will build a small a application that will evaluate a RPN string for the correct calculations. We only worry about adding, subtracting and multiplying.
calculator.evaluate('-1') #=> -1
calculator.evaluate('1 1 +') #=> 2
calculator.evaluate('1 -1 +') #=> 0
calculator.evaluate('2 2 *') #=> 2 * 2
calculator.evaluate('5 10 -') #=> 5 - 10
calculator.evaluate('1 2 3 4 + + +') #=> 1 + 2 + 3 + 4
calculator.evaluate('1 2 + 3 4 + *') #=> (2 + 1) * (4 + 3)
calculator.evaluate('20 10 5 4 + * -') #=> 20 - 10 * (5 + 4)