Infix-To-Postfix(using stack lib)

// InfixToPostfix.java
// Program to convert Infix expression to Postfix using Stack
// Time Complexity: O(n)
// Space Complexity: O(n)

import java.util.Stack;

public class InfixToPostfix {

    // Function to define operator precedence
    static int precedence(char op) {
        if (op == '+' || op == '-') return 1;
        if (op == '*' || op == '/') return 2;
        if (op == '^') return 3;
        return 0;
    }

    // Function to check if a character is an operator
    static boolean isOperator(char c) {
        return (c == '+' || c == '-' || c == '*' || c == '/' || c == '^');
    }

    // Function to convert infix to postfix
    static String infixToPostfix(String infix) {
        Stack<Character> stack = new Stack<>();
        StringBuilder postfix = new StringBuilder();

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

            // If operand, add it to output
            if (Character.isLetterOrDigit(c)) {
                postfix.append(c);
            }
            // If '(', push to stack
            else if (c == '(') {
                stack.push(c);
            }
            // If ')', pop until '('
            else if (c == ')') {
                while (!stack.isEmpty() && stack.peek() != '(') {
                    postfix.append(stack.pop());
                }
                stack.pop(); // remove '('
            }
            // If operator
            else if (isOperator(c)) {
                while (!stack.isEmpty() && precedence(stack.peek()) >= precedence(c)) {
                    postfix.append(stack.pop());
                }
                stack.push(c);
            }
        }

        // Pop remaining operators
        while (!stack.isEmpty()) {
            postfix.append(stack.pop());
        }

        return postfix.toString();
    }

    // Main function
    public static void main(String[] args) {
        String infix = "A+B*C-D";
        System.out.println("Infix Expression: " + infix);
        String postfix = infixToPostfix(infix);
        System.out.println("Postfix Expression: " + postfix);
    }
}