1. Read Postfix Expression from left to right element by element
2.   if the element is an operand (number of variable), push it into the stack
3.   if the token is an operator, 
       3.1   pop the two top most operands from the stack
       3.2   apply the binary operator with the two operands
       3.3    push the result into the stack
4. At the end, the stack should have only one value that is the result of the expression