package com.thealgorithms.stacks;
import java.util.Stack;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public final class InfixToPostfix {
private InfixToPostfix() {
}
public static String infix2PostFix(String infixExpression) throws Exception {
if (!BalancedBrackets.isBalanced(filterBrackets(infixExpression))) {
throw new Exception("invalid expression");
}
StringBuilder output = new StringBuilder();
Stack<Character> stack = new Stack<>();
for (char element : infixExpression.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.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("");
}
}