/** Evaluating postfix expression.
    @author Jed Yang, 2018-01-19
*/
import java.util.List;
import java.util.ArrayList;
import java.util.Scanner;

// Stack: push/pop/peek/isEmpty/EmptyStackException
// Queue: add/remove/element/isEmpty/NoSuchElementException
public class Postfix
{
   /** Evaluates a correct space-delimited postfix expression.
       Note: no error checking -- assumes expression is corretly formatted.
   */
   public static int evaluatePostfix(String expression)
   {
      Stack<Integer> stack = new CarlStack<Integer>();
      String[] array = expression.split(" ");
      for (String s : array)
      {
         if (s.equals("+"))
         {
            int secondNum = stack.pop();
            int firstNum = stack.pop();
            int answer = firstNum + secondNum;
            stack.push(answer);
         }
         else if (s.equals("-"))
         {
            int secondNum = stack.pop();
            int firstNum = stack.pop();
            int answer = firstNum - secondNum;
            stack.push(answer);
         }
         else if (s.equals("*"))
         {
            int secondNum = stack.pop();
            int firstNum = stack.pop();
            int answer = firstNum * secondNum;
            stack.push(answer);
         }
         else if (s.equals("/"))
         {
            int secondNum = stack.pop();
            int firstNum = stack.pop();
            int answer = firstNum / secondNum;
            stack.push(answer);
         }
         else
         {
            int number = Integer.parseInt(s);
            stack.push(number);
         }
      }
      int answer = stack.pop();
      return answer;
   }

   public static void main(String[] args)
   {
      Scanner kb = new Scanner(System.in);
      System.out.print("Postfix expression, please: ");
      String expression = kb.nextLine();
//      expression = "1 2 + 3 4 - 5 6 * - 7 8 9 * - * +";
      System.out.println(evaluatePostfix(expression));
   }
}