// CSE 143, Winter 2010, Marty Stepp
// 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
        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);   // "5 2 4 * + 7 -"
        while (input.hasNext()) {
            if (input.hasNextInt()) {
                // an operand; a number like 5 2 4.  push it onto the stack
                s.push(input.nextInt());
            } else {
                // an operator + - *
                int right = s.pop();
                int left = s.pop();
                String operator = input.next();   // "+", "*"
                // left operator right

                if (operator.equals("+")) {
                    s.push(left + right);
                } else if (operator.equals("-")) {
                    s.push(left - right);
                } else if (operator.equals("*")) {
                    s.push(left * right);
                } else {  // operator.equals("/")
                    s.push(left / right);
                }
            }
        }
        
        return s.pop();  // last remaining element on stack is the answer
    }
}