// CSE 143, Autumn 2013
// 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 + +"));           // 6
        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> s = new Stack<Integer>();
        Scanner input = new Scanner(expression);
        
        while(input.hasNext()) {
        	if(input.hasNextInt()) {
        		s.push(input.nextInt());
        	} else {
        		String op = input.next();
        		int operand2 = s.pop();
        		int operand1 = s.pop();
        		if(op.equals("+")) {
        			s.push(operand1 + operand2);
        		} else if(op.equals("-")) {
        			s.push(operand1 - operand2);
        		} else if(op.equals("*")) {
        			s.push(operand1 * operand2);
        		} else if(op.equals("/")) {
        			s.push(operand1 / operand2);
        		}
        	}
        }
        return s.pop();
    }
}