Algorithm for evaluating Postfix expressions:

(1)	Create a stack for storing operands and intermediate results

(2)	Scan the expression left-to-right

(3)	If the symbol is an operand, push it on the stack

(4)	If the symbol is an operator, pop the top two values off the stack 
    and perform the operator (first popped is right operand, second 
    popped is left operand)

       a. If the stack doesn't have two values, the expression is invalid

(5)	After processing the entire expression, the stack should have one 
    value left on it which is the answer

       a. If the stack is empty or has more than one value, the expression 
          is invalid