# Challenge: Evaluate Postfix Expressions Using Stacks

Let's try to compute postfix mathematical expressions using stacks!

We'll cover the following

## Infix and Postfix Expressions #

In the infix expression (the usual convention followed in mathematics), operators like + and * appear between the operands involved in the calculation:

6 + 3 * 8 - 4


Another convention is the postfix expression, where the operators appear after the operands involved in the expression. In postfix, the expression written above will become:

6 3 8 * + 4 -


The two operands 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.

## Problem Statement #

In this problem, you have to implement the evaluatePostFix() method that will compute a postfix expression given to it in a string.

### Input #

The input is a string containing a valid postfix mathematic expression. Each digit is considered a separate number, i.e., there are no double-digit numbers.

### Output #

An integer result of the given postfix expression.

### Sample Input #

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


### Sample Output #

3


#### Explanation #

1st operation => 2 * 1 = 2,
2nd operation => 9 - 2 = 7,
3rd operation => 7 - 8 = -1,
4th operation => -1 + 4 = 3

Level up your interview prep. Join Educative to access 70+ hands-on prep courses.