Extra point No. 2: Postfix calculator (2.5 points in the final grade that can
apply to either programming assignment or exam, Due: Oct. 30, 11:59pm)

We have learnt the algorithm to evaluate postfix expressions. By using the 
stack data structure, evaluating a postfix expression is mechanic and simple: 
when seeing an operand, push the operand to stack; when seeing an operator 
(+, -, *, / in this program), get two operands from the top of the stack, 
perform the operation, and push the result back to the stack. At the end,
the result is on the top of the stack. 

This program (postfix calculator) reads inputs line by line. Each line 
contains a correct postfix expression, which consists of integer numbers 
(operands) and operators (+, -, *, /). The calculator outputs the postfix 
expression and the evaluation result. The tokens in a postfix expression 
are separated by space. See given the sample executable (extra2.x) and 
test file.

To run, use command './extra2.x < extra2_test.txt'

Note: stringstream is a good way to get tokens out of an expression. 
Following is the code snippet that uses stringstream type to get tokens
in the expression:

        stringstream ll(line);
        string token; 
        while (ll >> token) {
	        // process each token
        }