// CSE 143, Winter 2012
// This program evaluates "postfix" expressions (also called "Reverse Polish 
// Notation"), which are mathematical expressions but with the operators placed
// after operands instead of between.
// For example: 1 + 2 * 3 + 4  is written as 1 2 3 * + 4 +

import java.util.*;

public class Postfix {
    public static void main(String[] args) {
        System.out.println(postfixEvaluate("1 2 3 +"));             // 3
        System.out.println(postfixEvaluate("1 2 3 * + 4 +"));       // 11
        System.out.println(postfixEvaluate("5 2 4 * + 7 -"));       // 6
        System.out.println(postfixEvaluate("2 3 + 4 5 * +"));       // 25
        System.out.println(postfixEvaluate("8 5 * 7 4 2 + * +"));   // 82
        System.out.println(postfixEvaluate("6 8 2 / 1 - *"));       // 18
    }
    
    // Evaluates the given postfix expression string and returns the result.
    // Precondition: expression consists only of integers, +-*/, and spaces in
    //               proper postfix format such as "2 3 - 4 *"
    public static int postfixEvaluate(String expression) {
        Stack<Integer> stack = new Stack<Integer>();
        Scanner tokens = new Scanner(expression);
        while (tokens.hasNext()) {
            if (tokens.hasNextInt()) {
                // operand (number)
                int num = tokens.nextInt();
                stack.push(num);
            } else {
                // operator (string)
                String operator = tokens.next();   // "*"  "-"
                int num2 = stack.pop();
                int num1 = stack.pop();
                
                int result;
                if (operator.equals("+")) {
                    result = num1 + num2;
                } else if (operator.equals("-")) {
                    result = num1 - num2;
                } else if (operator.equals("*")) {
                    result = num1 * num2;
                } else {  // if (operator.equals("/")) {
                    result = num1 / num2;
                }
                stack.push(result);
            }
        }
        
        // there SHOULD be exactly 1 thing on the stack now
        return stack.pop();
    }
}