import java.util.ArrayList;
import java.util.Scanner;


public class Postfix {

        // use evalStack for the evaluation of the postfix expression in line
	// implement evalStack with the pop and push methods
	private ArrayList<Integer> evalStack = new ArrayList<Integer>();
	
	// when scanning a next Token, put it in nextToken
	private String nextToken;
	
	// scLine is the scanner that gets the input line
	private Scanner scLine;
	
	// debug is used to report progress
	private boolean debug;
	
	/* constructor
	 * set instance variable debug
	 * create new scanner with String line and assign to scLine
	*/
	public Postfix(String line, boolean debug){
		this.debug = debug;
		scLine = new Scanner(line);
	}

	
	/*
	 * push Integer i on evalStack
	 * In debug mode print "push: " + pushed value
	 */
	private void push(Integer i){
	}
	
	/*
	 * pop Integer from evalStack
	 * If evalStack empty, return null
	 * In debug mode print "pop: " + popped value
	 */
	private Integer pop(){
		return null;
	}
	
	
	/*
	 * evaluate the postfix expression in scLine
	 * when encountering an Integer, push it on the evalStack
	 * when encountering an operator, 
	 *    pop two values off the evalStack (right, then left)
	 *    perform the operation, and push the result on the evalStack
	 * at the end of the expression,
	 *    pop the result off the stack and return it 
	 *    
	 * scLine either contains an empty line, or a valid postfix expression     
	 */
	public Integer eval(){
		return null;
	}
}