[ADS] Postfix notation

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.NoSuchElementException;


interface Stack<E> {

    // The elements of the Stack are any kind of objects

    // Access methods:

    public boolean isEmpty ();
        // Returns true only if the stack is empty.

    public E peek ();
        // Returns the element on the top od the stack.

    // Transformation methods:

    public void clear ();
        // Clears the stack.

    public void push (E x);
        // Adds x on the top of the stack.

    public E pop ();
        // Removes and returns the element on the top.
}

class ArrayStack<E> implements Stack<E> {
    private E[] elems;
    private int depth;

    @SuppressWarnings("unchecked")
    public ArrayStack (int maxDepth) {
        // Creating new empty stack
        elems = (E[]) new Object[maxDepth];
        depth = 0;
    }


    public boolean isEmpty () {
         // Returns true only if the stack is empty.

        return (depth == 0);
    }


    public E peek () {
        // Returns the element on the top od the stack.
        if (depth == 0)
            throw new NoSuchElementException();
        return elems[depth-1];
    }


    public void clear () {
        // Clears the stack.
        for (int i = 0; i < depth; i++)  elems[i] = null;
        depth = 0;
    }


    public void push (E x) {
        // Adds x on the top of the stack.
        elems[depth++] = x;
    }


    public E pop () {
        // Removes and returns the element on the top.
        if (depth == 0)
            throw new NoSuchElementException();
        E topmost = elems[--depth];
        elems[depth] = null;
        return topmost;
    }
}

public class PostFixEvaluation {

    static boolean Operation (char znak){
        if(znak =='+' || znak == '-' || znak =='/' || znak== '*'){
            return true;
        }
        return false;
    }

    static int evaluatePostfix(char [] izraz, int n)
    {

        ArrayStack<Integer> integers = new ArrayStack <>(n);
        ArrayStack<Character> operators = new ArrayStack<>(n);

        StringBuffer sb = new StringBuffer();

        for(int i = 0 ; i <n ; i ++){
            if(Character.isDigit(izraz[i])){
                if(Character.isDigit(izraz[i+1])){
                    sb.append(izraz[i]);
                    continue;
                }
                else{
                    sb.append(izraz[i]);
                    integers.push(Integer.parseInt(sb.toString()));
                    sb.setLength(0);
                }
            }

            int rezultat = 0 ;
            if(Operation(izraz[i])){
                char znak = izraz[i];
                {
                    if(znak == '-'){
                        int tmp = integers.pop();
                        rezultat = integers.pop()-tmp;
                    }else if(znak == '+'){
                        int tmp = integers.pop();
                        rezultat = integers.pop()+tmp;
                    }else if(znak == '/'){
                        int tmp = integers.pop();
                        rezultat = integers.pop()/tmp;
                    }else if(znak == '*'){
                        int tmp = integers.pop();
                        rezultat = integers.pop()*tmp;
                    }
                  }
                  integers.push(rezultat);
            }

        }
        return integers.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();

        int rez = evaluatePostfix(exp, exp.length);
        System.out.println(rez);

        br.close();

    }

}