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Hardvanalxkm
Hardvan
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alxkm
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Add Infix To Prefix new algorithm with unit tests (TheAlgorithms#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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‎DIRECTORY.md

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* greedyalgorithms
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* [ActivitySelection](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/greedyalgorithms/ActivitySelection.java)
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* [CoinChange](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/greedyalgorithms/CoinChange.java)
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* [DigitSeparation](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/greedyalgorithms/DigitSeparation.java)
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* [FractionalKnapsack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/greedyalgorithms/FractionalKnapsack.java)
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* [GaleShapley](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/greedyalgorithms/GaleShapley.java)
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* [JobSequencing](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/greedyalgorithms/JobSequencing.java)
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* [DecimalToAnyUsingStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/DecimalToAnyUsingStack.java)
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* [DuplicateBrackets](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/DuplicateBrackets.java)
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* [InfixToPostfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/InfixToPostfix.java)
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* [InfixToPrefix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/InfixToPrefix.java)
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* [LargestRectangle](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/LargestRectangle.java)
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* [MaximumMinimumWindow](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/MaximumMinimumWindow.java)
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* [NextGreaterElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextGreaterElement.java)
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* greedyalgorithms
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* [ActivitySelectionTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/greedyalgorithms/ActivitySelectionTest.java)
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* [CoinChangeTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/greedyalgorithms/CoinChangeTest.java)
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* [DigitSeparationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/greedyalgorithms/DigitSeparationTest.java)
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* [FractionalKnapsackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/greedyalgorithms/FractionalKnapsackTest.java)
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* [GaleShapleyTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/greedyalgorithms/GaleShapleyTest.java)
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* [JobSequencingTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/greedyalgorithms/JobSequencingTest.java)
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* [DecimalToAnyUsingStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/DecimalToAnyUsingStackTest.java)
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* [DuplicateBracketsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/DuplicateBracketsTest.java)
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* [InfixToPostfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/InfixToPostfixTest.java)
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* [InfixToPrefixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/InfixToPrefixTest.java)
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* [LargestRectangleTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/LargestRectangleTest.java)
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* [NextGreaterElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextGreaterElementTest.java)
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* [NextSmallerElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextSmallerElementTest.java)

‎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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}

‎src/test/java/com/thealgorithms/stacks/InfixToPrefixTest.java

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package com.thealgorithms.stacks;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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import static org.junit.jupiter.api.Assertions.assertThrows;
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import java.util.stream.Stream;
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import org.junit.jupiter.api.Test;
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import org.junit.jupiter.params.ParameterizedTest;
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import org.junit.jupiter.params.provider.Arguments;
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import org.junit.jupiter.params.provider.MethodSource;
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public class InfixToPrefixTest {
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@ParameterizedTest
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@MethodSource("provideValidExpressions")
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void testValidExpressions(String infix, String expectedPrefix) throws Exception {
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assertEquals(expectedPrefix, InfixToPrefix.infix2Prefix(infix));
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}
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@Test
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void testEmptyString() {
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// Assuming that an empty string returns an empty prefix or throws an exception
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assertEquals("", InfixToPrefix.infix2Prefix(""));
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}
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@Test
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void testNullValue() {
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// Assuming that a null input throws a NullPointerException
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assertThrows(NullPointerException.class, () -> InfixToPrefix.infix2Prefix(null));
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}
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private static Stream<Arguments> provideValidExpressions() {
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return Stream.of(Arguments.of("3+2", "+32"), // Simple addition
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Arguments.of("1+(2+3)", "+1+23"), // Parentheses
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Arguments.of("(3+4)*5-6", "-*+3456"), // Nested operations
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Arguments.of("a+b*c", "+a*bc"), // Multiplication precedence
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Arguments.of("a+b*c/d", "+a/*bcd"), // Division precedence
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Arguments.of("a+b*c-d", "-+a*bcd"), // Subtraction precedence
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Arguments.of("a+b*c/d-e", "-+a/*bcde"), // Mixed precedence
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Arguments.of("a+b*(c-d)", "+a*b-cd"), // Parentheses precedence
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Arguments.of("a+b*(c-d)/e", "+a/*b-cde") // Mixed precedence with parentheses
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);
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}
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}

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