diff --git a/DIRECTORY.md b/DIRECTORY.md index 4fb916996aac..3662a8cda5c2 100644 --- a/DIRECTORY.md +++ b/DIRECTORY.md @@ -552,6 +552,7 @@ * [NextGreaterElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextGreaterElement.java) * [NextSmallerElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextSmallerElement.java) * [PostfixToInfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PostfixToInfix.java) + * [PrefixToInfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PrefixToInfix.java) * [StackPostfixNotation](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/StackPostfixNotation.java) * strings * [AhoCorasick](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/AhoCorasick.java) @@ -984,6 +985,7 @@ * [NextGreaterElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextGreaterElementTest.java) * [NextSmallerElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextSmallerElementTest.java) * [PostfixToInfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PostfixToInfixTest.java) + * [PrefixToInfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PrefixToInfixTest.java) * [StackPostfixNotationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/StackPostfixNotationTest.java) * strings * [AhoCorasickTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/AhoCorasickTest.java) diff --git a/src/main/java/com/thealgorithms/stacks/PrefixToInfix.java b/src/main/java/com/thealgorithms/stacks/PrefixToInfix.java new file mode 100644 index 000000000000..41eb974b0e5b --- /dev/null +++ b/src/main/java/com/thealgorithms/stacks/PrefixToInfix.java @@ -0,0 +1,69 @@ +package com.thealgorithms.stacks; + +import java.util.Stack; + +/** + * Converts a prefix expression to an infix expression using a stack. + * + * The input prefix expression should consist of + * valid operands (letters or digits) and operators (+, -, *, /, ^). + * Parentheses are not required in the prefix string. + */ +public final class PrefixToInfix { + private PrefixToInfix() { + } + + /** + * Determines if a given character is a valid arithmetic operator. + * + * @param token the character to check + * @return true if the character is an operator, false otherwise + */ + public static boolean isOperator(char token) { + return token == '+' || token == '-' || token == '/' || token == '*' || token == '^'; + } + + /** + * Converts a valid prefix expression to an infix expression. + * + * @param prefix the prefix expression to convert + * @return the equivalent infix expression + * @throws NullPointerException if the prefix expression is null + */ + public static String getPrefixToInfix(String prefix) { + if (prefix == null) { + throw new NullPointerException("Null prefix expression"); + } + if (prefix.isEmpty()) { + return ""; + } + + Stack stack = new Stack<>(); + + // Iterate over the prefix expression from right to left + for (int i = prefix.length() - 1; i >= 0; i--) { + char token = prefix.charAt(i); + + if (isOperator(token)) { + // Pop two operands from stack + String operandA = stack.pop(); + String operandB = stack.pop(); + + // Form the infix expression with parentheses + String infix = "(" + operandA + token + operandB + ")"; + + // Push the resulting infix expression back onto the stack + stack.push(infix); + } else { + // Push operand onto stack + stack.push(Character.toString(token)); + } + } + + if (stack.size() != 1) { + throw new ArithmeticException("Malformed prefix expression"); + } + + return stack.pop(); // final element on the stack is the full infix expression + } +} diff --git a/src/test/java/com/thealgorithms/stacks/PrefixToInfixTest.java b/src/test/java/com/thealgorithms/stacks/PrefixToInfixTest.java new file mode 100644 index 000000000000..83fd09e1bbf6 --- /dev/null +++ b/src/test/java/com/thealgorithms/stacks/PrefixToInfixTest.java @@ -0,0 +1,44 @@ +package com.thealgorithms.stacks; + +import static org.junit.jupiter.api.Assertions.assertEquals; +import static org.junit.jupiter.api.Assertions.assertThrows; + +import java.util.stream.Stream; +import org.junit.jupiter.api.Test; +import org.junit.jupiter.params.ParameterizedTest; +import org.junit.jupiter.params.provider.Arguments; +import org.junit.jupiter.params.provider.MethodSource; + +class PrefixToInfixTest { + + @ParameterizedTest + @MethodSource("provideValidPrefixToInfixTestCases") + void testValidPrefixToInfixConversion(String prefix, String expectedInfix) { + assertEquals(expectedInfix, PrefixToInfix.getPrefixToInfix(prefix)); + } + + static Stream provideValidPrefixToInfixTestCases() { + return Stream.of(Arguments.of("A", "A"), // Single operand + Arguments.of("+AB", "(A+B)"), // Addition + Arguments.of("*+ABC", "((A+B)*C)"), // Addition and multiplication + Arguments.of("-+A*BCD", "((A+(B*C))-D)"), // Mixed operators + Arguments.of("/-A*BC+DE", "((A-(B*C))/(D+E))"), // Mixed operators + Arguments.of("^+AB*CD", "((A+B)^(C*D))") // Mixed operators + ); + } + + @Test + void testEmptyPrefixExpression() { + assertEquals("", PrefixToInfix.getPrefixToInfix("")); + } + + @Test + void testNullPrefixExpression() { + assertThrows(NullPointerException.class, () -> PrefixToInfix.getPrefixToInfix(null)); + } + + @Test + void testMalformedPrefixExpression() { + assertThrows(ArithmeticException.class, () -> PrefixToInfix.getPrefixToInfix("+ABC")); + } +}