infixevaluation

import java.util.Scanner;
import java.util.Stack;

public class CSE220Assignment04 {
    public static void main(String[] args) {
        String exp=new Scanner(System.in).next();
        String postFixExp = postFixBuilder(exp);
        System.out.println("Post Fix Expression: "+postFixExp);
        int result = postFixEvaluator(add(postFixExp));
        System.out.println("Answer: "+result);
    }

    private static String add(String postFixExp) {
        String s ="";
        for (int i=0; i<postFixExp.length(); i++){
            s+=postFixExp.charAt(i)+" ";
        }
        return s;
    }
    public static String postFixBuilder(String exp){
        String result = "";

        Stack<Character> stack = new Stack<>();

        for (int i = 0; i<exp.length(); ++i)
        {
            char c = exp.charAt(i);

            if (Character.isLetterOrDigit(c))
                result += c;

            else if (c == '(')
                stack.push(c);

            else if (c == ')')
            {
                while (!stack.isEmpty() && stack.peek() != '(')
                    result += stack.pop();

                if (!stack.isEmpty() && stack.peek() != '(')
                    return "Invalid Expression";
                else
                    stack.pop();
            }
            else // an operator is encountered
            {
                while (!stack.isEmpty() && Prec(c) <= Prec(stack.peek()))
                    result += stack.pop();
                stack.push(c);
            }

        }
        while (!stack.isEmpty())
            result += stack.pop();

        return result;
    }
    static int Prec(char ch)
    {
        switch (ch)
        {
            case '+':
            case '-':
                return 1;

            case '*':
            case '/':
                return 2;

            case '^':
                return 3;
        }
        return -1;
    }
    public static int postFixEvaluator(String postfixExpr) {
        Stack<Integer> s = new Stack<Integer>();
        String[] items   = postfixExpr.split(" ");

        for (String item : items) {
            try {
                s.push(Integer.valueOf(item));
            } catch (NumberFormatException e) {
                Integer value1 = s.pop();
                Integer value2 = s.pop();

                switch (item) {
                    case "+":
                        s.push(value2 + value1);
                        break;
                    case "-":
                        s.push(value2 - value1);
                        break;
                    case "*":
                        s.push(value2 * value1);
                        break;
                    case "/":
                        s.push(value2 / value1);
                        break;
                }
            }
        }

        return s.pop();
    }
}