Постфикс нотација

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.lang.reflect.Array;
import java.util.NoSuchElementException;
import java.util.ArrayList;


interface Stack<E>{
    public E pop();
    public void clear();
    public E peek();
    public boolean isEmpty();
}

class SLLNode<E>{
    protected E data;
    protected SLLNode succ;
    SLLNode(){}
    SLLNode(E data, SLLNode<E> succ){
        this.data = data;
        this.succ = succ;
    }
}

class StackList<E> implements Stack{
    SLLNode<E> top;
    StackList(){
        top = null;
    }
    public void push(E data){
        SLLNode<E> ins = new SLLNode<>(data, top);
        top = ins;
    }
    public E pop(){
        if(top == null) System.out.print("Nema elementi vo stekot. ");
        E ret = top.data;
        top = top.succ;
        return ret;
    }

    public void clear(){top = null;}

    public E peek(){return top.data;}

    public boolean isEmpty(){return (top==null);}

}

public class PostFixEvaluation {

    public static int postfix(String [] s) throws Exception{
        StackList<Integer> stek = new StackList<>();
        for(String x : s){
            if(Character.isDigit(x.charAt(0)))
                stek.push(Integer.parseInt(x));

            else if(x.charAt(0) == '+')
                stek.push(stek.pop() + stek.pop());

            else if(x.charAt(0) == '-'){
                int vt = stek.pop();
                int pr = stek.pop();
                stek.push(pr - vt);
            }

            else if(x.charAt(0) == '/'){
                int vt = stek.pop();
                int pr = stek.pop();
                stek.push(pr / vt);
            }

            else if(x.charAt(0) == '*')
                stek.push(stek.pop() * stek.pop());
        }
        return stek.pop();
    }

    public static void main(String[] args) throws Exception{

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        String expression = br.readLine();
        char exp[] = expression.toCharArray();

        String [] ex = expression.split(" ");
        System.out.print(postfix(ex));

        br.close();

    }

}