4.Lab4.1 Postfix notation (Solved)

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Arrays;
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 isOperator(char sign) {
        if (sign == '+' || sign == '-' || sign == '*' || sign == '/') {
            return true;
        }
        return false;
    }

    static int evaluatePostfix(char symbols[], 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++) {

            /* Make sure you're getting the numbers and not just their digits */
            if (Character.isDigit(symbols[i])) {
                if (Character.isDigit(symbols[i + 1])) {
                    sb.append(symbols[i]);
                    continue;
                } else {
                    sb.append(symbols[i]);
                    integers.push(Integer.parseInt(sb.toString()));
                    sb.setLength(0);
                }
            }

            int result = 0;
            if (isOperator(symbols[i])) {
                char sign = symbols[i];
                if (sign == '-') {
                    int temp = integers.pop();
                    result = integers.pop() - temp;
                } else if (sign == '+') {
                    int temp = integers.pop();
                    result = integers.pop() + temp;
                } else if (sign == '/') {
                    int temp = integers.pop();
                    result = integers.pop() / temp;
                } else if (sign == '*') {
                    int temp = integers.pop();
                    result = integers.pop() * temp;
                }
                integers.push(result);
            }
        }

        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();

    }

}