CSCI-258-Labs/258/calculatorAlgorithm/Main.java

import java.lang.Integer;
import java.util.Scanner;
import java.util.Stack;

class Main {

    public static void main(String args[]) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter Equation : ");

        String postfix = infixToPostfix(input.nextLine());

        System.out.println(solve(postfix));

        input.close();
    }

    public static int solve(String postfix) {
        Stack<Integer> ints = new Stack<>();

        for (int i = 0; i < postfix.length(); i++) {
            char x = postfix.charAt(i);
            if (x == ' ') {
                continue;
            } else if (Character.isDigit(x)) {
                String fullNumber = "";

                while (postfix.charAt(i) != ' ') {
                    fullNumber += postfix.charAt(i);
                    i++;
                }

                ints.push(Integer.parseInt(fullNumber));
            } else {
                int num2 = ints.pop();
                int num1 = ints.pop();

                switch (x) {
                    case '/':
                        ints.push(num1 / num2);
                        break;
                    case '*':
                        ints.push(num1 * num2);
                        break;
                    case '+':
                        ints.push(num1 + num2);
                        break;
                    case '-':
                        ints.push(num1 - num2);
                        break;
                }
            }
        }

        return ints.pop();
    }

    public static Integer calculatePrecedence(Character c) {
        if (c == '*' || c == '/') {
            return 1;
        } else {
            return 0;
        }
    }

    public static String infixToPostfix(String infix) {
        Stack<Character> stack = new Stack<>();

        String postfix = "";

        int i = 0;

        while (i < infix.length()) {
            char c = infix.charAt(i);

            if (Character.isDigit(c)) {
                String num = "";

                while (
                    (i < infix.length()) && Character.isDigit(infix.charAt(i))
                ) {
                    num += infix.charAt(i);

                    i++;
                }

                postfix += num + " ";
            } else {
                while (
                    !stack.empty() &&
                    calculatePrecedence(stack.peek()) >= calculatePrecedence(c)
                ) {
                    postfix += stack.pop();
                }

                stack.push(c);

                i++;
            }
        }

        while (!stack.empty()) {
            postfix += stack.pop();
        }

        return postfix;
    }
}