package com.thealgorithms.stacks;
import java.util.Stack;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public final class InfixToPrefix {
private InfixToPrefix() {
}
public static String infix2Prefix(String infixExpression) throws IllegalArgumentException {
if (infixExpression == null) {
throw new NullPointerException("Input expression cannot be null.");
}
infixExpression = infixExpression.trim();
if (infixExpression.isEmpty()) {
return "";
}
if (!BalancedBrackets.isBalanced(filterBrackets(infixExpression))) {
throw new IllegalArgumentException("Invalid expression: unbalanced brackets.");
}
StringBuilder output = new StringBuilder();
Stack<Character> stack = new Stack<>();
String reversedInfix = new StringBuilder(infixExpression).reverse().toString();
for (char element : reversedInfix.toCharArray()) {
if (Character.isLetterOrDigit(element)) {
output.append(element);
} else if (element == ')') {
stack.push(element);
} else if (element == '(') {
while (!stack.isEmpty() && stack.peek() != ')') {
output.append(stack.pop());
}
stack.pop();
} else {
while (!stack.isEmpty() && precedence(element) < precedence(stack.peek())) {
output.append(stack.pop());
}
stack.push(element);
}
}
while (!stack.isEmpty()) {
output.append(stack.pop());
}
return output.reverse().toString();
}
private static int precedence(char operator) {
switch (operator) {
case '+':
case '-':
return 0;
case '*':
case '/':
return 1;
case '^':
return 2;
default:
return -1;
}
}
private static String filterBrackets(String input) {
Pattern pattern = Pattern.compile("[^(){}\\[\\]<>]");
Matcher matcher = pattern.matcher(input);
return matcher.replaceAll("");
}
}