Untitled

import java.util.*;

public class InfixToPostfix {
    
    // This method returns the precedence of the operator
    public static int getPrecedence(char ch) {
        switch(ch) {
            case '+':
            case '-':
                return 1;
                
            case '*':
            case '/':
                return 2;
                
            case '^':
                return 3;
        }
        return -1;
    }
    
    // This method converts an infix expression to postfix expression
    public static String infixToPostfix(String infix) {
        Stack<Character> stack = new Stack<>();
        StringBuilder postfix = new StringBuilder();
        
        for(int i = 0; i < infix.length(); i++) {
            char ch = infix.charAt(i);
            
            if(Character.isLetterOrDigit(ch)) {
                postfix.append(ch);
            } else if(ch == '(') {
                stack.push(ch);
            } else if(ch == ')') {
                while(!stack.isEmpty() && stack.peek() != '(') {
                    postfix.append(stack.pop());
                }
                stack.pop();
            } else {
                while(!stack.isEmpty() && getPrecedence(ch) <= getPrecedence(stack.peek())) {
                    postfix.append(stack.pop());
                }
                stack.push(ch);
            }
        }
        
        while(!stack.isEmpty()) {
            if(stack.peek() == '(') {
                return "Invalid Expression";
            }
            postfix.append(stack.pop());
        }
        
        return postfix.toString();
    }
    
    // This method evaluates a postfix expression
    public static int evaluatePostfix(String postfix) {
        Stack<Integer> stack = new Stack<>();
        
        for(int i = 0; i < postfix.length(); i++) {
            char ch = postfix.charAt(i);
            
            if(Character.isDigit(ch)) {
                stack.push(ch - '0');
            } else {
                int operand2 = stack.pop();
                int operand1 = stack.pop();
                int result = 0;
                switch(ch) {
                    case '+':
                        result = operand1 + operand2;
                        break;
                        
                    case '-':
                        result = operand1 - operand2;
                        break;
                        
                    case '*':
                        result = operand1 * operand2;
                        break;
                        
                    case '/':
                        result = operand1 / operand2;
                        break;
                        
                    case '^':
                        result = (int) Math.pow(operand1, operand2);
                        break;
                }
                stack.push(result);
            }
        }
        
        return stack.pop();
    }
    
    // Main method to test the program
    public static void main(String[] args) {
        String infix = "2+3*4-5";
        String postfix = infixToPostfix(infix);
        System.out.println("Postfix notation: " + postfix);
        
        int result = evaluatePostfix(postfix);
        System.out.println("Result: " + result);
    }
}