PolishNotation.java

import java.util.Stack;

public class PolishNotation { // also known as prefix notation
    public static void main(String[] args)
    {                                                       // * / 12 3 25
        String[] input = {"*","/", "12", "3", "25"};        //  * ( 12 / 3 ) 25
        System.out.println(getPolishSum(input));            // ( 12 / 3 ) * 25
    }                                                       // 100

    private static long getPolishSum(String[] input) {
        Stack<String> digitsStack = new Stack<String>();
        for (int i =input.length-1; i > -1; i--) {
            if (input[i].equals("+") ||
                input[i].equals("-") ||
                input[i].equals("*") ||
                input[i].equals("/") ) {
                long firstValue = Long.valueOf(digitsStack.pop());
                long secondValue = Long.valueOf(digitsStack.pop());
                long temp = 0;
                if (input[i].equals("+")) temp = firstValue + secondValue;
                else if (input[i].equals("-")) temp = firstValue - secondValue;
                else if (input[i].equals("*")) temp = firstValue * secondValue;
                else if (input[i].equals("/")) temp = firstValue / secondValue;
                digitsStack.push(String.valueOf(temp));
            }
            else digitsStack.push(input[i]);
        }
        return Long.valueOf(digitsStack.pop());
    }
}