Calculate the value of a given expression in Reverse Polish Notation.
Definitions:
{expr} = [ {expr} {expr} {operator} | {integer} ]
operand = {integer}
operator = [+, -, /, *]
val_of(expr) := int_val(expr) if expr is integer else val_of(operand) x val_of(operand) where x is the operator
Ex 0:
RPN(9) = 9
RPN(3 5 +) = 8
RPN(3 1 + 2 +) = RPN(3 1 +) + RPN(2) = 4 + 2 = 10
Input Format:
Input expr is a list of strings where elements are either string of integers (ex: 3, 5) or a arithmetic operation sign (+, -, /, *).
Plase, note that results of each individual operation are guaranteed to be integers.
Ex 1:
RPN(["5", "2", "+", "6", "*"]) = ((5 + 2) * 6) = 42
Ex 2:
RPN([ "3", "4","*","1", "2", "+", "-"]) = (3 * 4) - (1 + 2) = 12 - 3 = 9