Challenge: Evaluate Postfix Expression Using a Stack

Problem Statement

The usual convention followed in mathematics is the infix expression. Operators like + and * appear between the two numbers involved in the calculation:

6 + 3 * 8 - 4

Another convention is the postfix expression where the operators appear after the two numbers involved in the expression. In postfix, the expression written above will be presented as:

6 3 8 * + 4 -

The two digits preceding an operator will be used with that operator

1. From the first block of digits 6 3 8, we pick the last two which are 3 and 8.
2. Reading the operators from left to right, the first one is *. The expression now becomes 3 * 8
3. The next number is 6 while the next operator is +, so we have 6 + 8 * 3.
4. The value of this expression is followed by 4, which is right before -. Hence we have 6 + 8 * 3 - 4.

Implement a function called evaluatePostFix() that will compute a postfix expression given to it as a string.

Input

A string containing a postfix mathematic expression. Each digit is considered to be a separate number, i.e., there are no double digit numbers.

Output

A result of the given postfix expression.

Sample Input

exp = "921 * - 8 - 4 +" # 9 - 2 * 1 - 8 + 4

Sample Output

3