Polish

PROBLEM STATEMENT

In Polish notation, an expression consists of an operator followed by its
operands.  For example, consider the following expressions in standard notation:

5 * 2 + -3
5 * (2 + -3)

The Polish notation equivalents of these expressions, respectively, are: 

+ * 5 2 -3
* 5 + 2 -3

Implement a class, Polish, which contains a method, evaluate, that calculates
an expression which is given in valid Polish notation.

DEFINITION
Class name: Polish
Method name: evaluate
Parameters: String
Return type: int

Method signature: int evaluate(String expr); Be sure your method is public.

TopCoder will ensure the input is valid.  Inputs are valid if the following
criteria are met:
* The String is given in valid Polish notation.  An expression is in valid
Polish notation if:
   - there is exactly one less operator than the number of operands
   - the first expression is not an operand
   - the last two expressions are operands
* An expression will not be more than 21 characters.
* Operands will be between -100 and 100, inclusive.
* Operators can only be a plus ('+'), minus ('-') or multiplication sign ('*').

NOTES
* There will be at least one space between all operands and operators in the
input string.
* There may be leading and trailing spaces in the input string.
* Unary minus signs should be considered to be part of the integer values, not
operators, and will not be separated from the values they modify by a space. 

EXAMPLES

1. Given the expression "+ * 5 2 -3" as input, the integer value 7 will be
returned.

2. Given the expression "* 5 + 2 -3" as input, the integer value -5 will be
returned.

3. Given the expression "+ * - + 24 5 29 5 -7" as input, the integer value -7
will be returned.