// CSE 143, Winter 2009, 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(evaluate("1 2 +"));              // 3
        System.out.println(evaluate("1 2 3 * + 4 +"));      // 11
        System.out.println(evaluate("2 3 + 4 5 * +"));      // 25
        System.out.println(evaluate("8 5 * 7 4 2 + * +"));  // 82
    }
    
    // Evaluates the given prefix expression string and returns its result.
    // Precondition: expression string represents a legal postfix expression
    //               containing only integers and + and *
    public static int evaluate(String expression) {
        Stack<Integer> s = new Stack<Integer>();
        Scanner input = new Scanner(expression);
        while (input.hasNext()) {
            if (input.hasNextInt()) {
                // an operand (a number); push it onto the stack
                s.push(input.nextInt());
            } else {
                // an operator (+ or *); apply it to the top two operands on the stack
                String operator = input.next();  // "+" , "*"
                int operand2 = s.pop();
                int operand1 = s.pop();
                if (operator.equals("+")) {
                    s.push(operand1 + operand2);
                } else {
                    s.push(operand1 * operand2);
                }
            }
        }
        return s.pop();
    }
}