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Add Infix To Prefix
new algorithm with unit tests (#5537)
* Add `Infix To Prefix` new algorithm * Update directory * Update directory * Fix clang * Fix clang * Add more tests * Fix comma error * Fix test cases * Fix comment * Remove unused import * Update directory * Add tests for null & empty strings * Implement suggested changes * Update directory * Fix comment --------- Co-authored-by: Hardvan <Hardvan@users.noreply.github.com> Co-authored-by: Alex Klymenko <alexanderklmn@gmail.com>
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92
src/main/java/com/thealgorithms/stacks/InfixToPrefix.java
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92
src/main/java/com/thealgorithms/stacks/InfixToPrefix.java
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package com.thealgorithms.stacks;
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import java.util.Stack;
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import java.util.regex.Matcher;
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import java.util.regex.Pattern;
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public final class InfixToPrefix {
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private InfixToPrefix() {
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}
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/**
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* Convert an infix expression to a prefix expression using stack.
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*
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* @param infixExpression the infix expression to convert
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* @return the prefix expression
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* @throws IllegalArgumentException if the infix expression has unbalanced brackets
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* @throws NullPointerException if the infix expression is null
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*/
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public static String infix2Prefix(String infixExpression) throws IllegalArgumentException {
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if (infixExpression == null) {
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throw new NullPointerException("Input expression cannot be null.");
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}
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infixExpression = infixExpression.trim();
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if (infixExpression.isEmpty()) {
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return "";
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}
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if (!BalancedBrackets.isBalanced(filterBrackets(infixExpression))) {
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throw new IllegalArgumentException("Invalid expression: unbalanced brackets.");
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}
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StringBuilder output = new StringBuilder();
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Stack<Character> stack = new Stack<>();
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// Reverse the infix expression for prefix conversion
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String reversedInfix = new StringBuilder(infixExpression).reverse().toString();
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for (char element : reversedInfix.toCharArray()) {
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if (Character.isLetterOrDigit(element)) {
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output.append(element);
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} else if (element == ')') {
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stack.push(element);
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} else if (element == '(') {
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while (!stack.isEmpty() && stack.peek() != ')') {
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output.append(stack.pop());
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}
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stack.pop();
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} else {
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while (!stack.isEmpty() && precedence(element) < precedence(stack.peek())) {
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output.append(stack.pop());
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}
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stack.push(element);
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}
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}
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while (!stack.isEmpty()) {
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output.append(stack.pop());
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}
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// Reverse the result to get the prefix expression
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return output.reverse().toString();
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}
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/**
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* Determines the precedence of an operator.
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*
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* @param operator the operator whose precedence is to be determined
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* @return the precedence of the operator
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*/
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private static int precedence(char operator) {
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switch (operator) {
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case '+':
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case '-':
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return 0;
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case '*':
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case '/':
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return 1;
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case '^':
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return 2;
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default:
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return -1;
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}
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}
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/**
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* Filters out all characters from the input string except brackets.
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*
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* @param input the input string to filter
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* @return a string containing only brackets from the input string
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*/
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private static String filterBrackets(String input) {
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Pattern pattern = Pattern.compile("[^(){}\\[\\]<>]");
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Matcher matcher = pattern.matcher(input);
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return matcher.replaceAll("");
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}
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}
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