diff --git a/src/main/java/com/thealgorithms/stacks/InfixToPrefix.java b/src/main/java/com/thealgorithms/stacks/InfixToPrefix.java
index 3d90d14e0d1e..e9ee5e208df6 100644
--- a/src/main/java/com/thealgorithms/stacks/InfixToPrefix.java
+++ b/src/main/java/com/thealgorithms/stacks/InfixToPrefix.java
@@ -4,85 +4,109 @@
import java.util.regex.Matcher;
import java.util.regex.Pattern;
+/**
+ * Utility class for converting an infix arithmetic expression
+ * into its equivalent prefix notation expression.
+ *
+ * This class provides a static method to perform the conversion,
+ * validating balanced brackets before processing.
+ *
+ */
public final class InfixToPrefix {
+
private InfixToPrefix() {
}
/**
- * Convert an infix expression to a prefix expression using stack.
+ * Converts a given infix expression string to a prefix expression string.
+ *
+ * The method validates that the input expression has balanced brackets using
+ * {@code BalancedBrackets.isBalanced} on the filtered bracket characters.
+ * It throws an {@code IllegalArgumentException} if the brackets are unbalanced,
+ * and a {@code NullPointerException} if the input is null.
+ *
+ *
+ * Supported operators: {@code +, -, *, /, ^} and operands can be letters or digits.
+ *
*
- * @param infixExpression the infix expression to convert
- * @return the prefix expression
- * @throws IllegalArgumentException if the infix expression has unbalanced brackets
- * @throws NullPointerException if the infix expression is null
+ * @param infixExpression the arithmetic expression in infix notation
+ * @return the equivalent prefix notation expression
+ * @throws IllegalArgumentException if brackets are unbalanced
+ * @throws NullPointerException if the input expression is null
*/
- public static String infix2Prefix(String infixExpression) throws IllegalArgumentException {
+ public static String infix2Prefix(String infixExpression) {
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 stack = new Stack<>();
- // Reverse the infix expression for prefix conversion
+ Stack operatorStack = new Stack<>();
+
+ // Reverse the infix expression to facilitate prefix conversion
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());
+
+ for (char token : reversedInfix.toCharArray()) {
+ if (Character.isLetterOrDigit(token)) {
+ // Append operands directly to output
+ output.append(token);
+ } else if (token == ')') {
+ // Push ')' onto stack (since expression is reversed, '(' and ')' roles swapped)
+ operatorStack.push(token);
+ } else if (token == '(') {
+ // Pop operators until ')' is found
+ while (!operatorStack.isEmpty() && operatorStack.peek() != ')') {
+ output.append(operatorStack.pop());
}
- stack.pop();
+ operatorStack.pop(); // Remove the ')'
} else {
- while (!stack.isEmpty() && precedence(element) < precedence(stack.peek())) {
- output.append(stack.pop());
+ // Pop operators with higher precedence before pushing current operator
+ while (!operatorStack.isEmpty() && precedence(token) < precedence(operatorStack.peek())) {
+ output.append(operatorStack.pop());
}
- stack.push(element);
+ operatorStack.push(token);
}
}
- while (!stack.isEmpty()) {
- output.append(stack.pop());
+
+ // Append any remaining operators in stack
+ while (!operatorStack.isEmpty()) {
+ output.append(operatorStack.pop());
}
- // Reverse the result to get the prefix expression
+ // Reverse the output to obtain the final prefix expression
return output.reverse().toString();
}
/**
- * Determines the precedence of an operator.
+ * Returns the precedence level of the given operator.
*
- * @param operator the operator whose precedence is to be determined
- * @return the precedence of the operator
+ * @param operator the operator character (e.g., '+', '-', '*', '/', '^')
+ * @return the precedence value: higher means higher precedence,
+ * or -1 if the character is not a recognized operator
*/
private static int precedence(char operator) {
- switch (operator) {
- case '+':
- case '-':
- return 0;
- case '*':
- case '/':
- return 1;
- case '^':
- return 2;
- default:
- return -1;
- }
+ return switch (operator) {
+ case '+', '-' -> 0;
+ case '*', '/' -> 1;
+ case '^' -> 2;
+ default -> -1;
+ };
}
/**
- * Filters out all characters from the input string except brackets.
+ * Extracts only the bracket characters from the input string.
+ * Supports parentheses (), curly braces {}, square brackets [], and angle brackets <>.
*
- * @param input the input string to filter
- * @return a string containing only brackets from the input string
+ * @param input the original expression string
+ * @return a string containing only bracket characters from the input
*/
private static String filterBrackets(String input) {
Pattern pattern = Pattern.compile("[^(){}\\[\\]<>]");
diff --git a/src/test/java/com/thealgorithms/stacks/InfixToPrefixTest.java b/src/test/java/com/thealgorithms/stacks/InfixToPrefixTest.java
index 91be8a63da62..0ea948307336 100644
--- a/src/test/java/com/thealgorithms/stacks/InfixToPrefixTest.java
+++ b/src/test/java/com/thealgorithms/stacks/InfixToPrefixTest.java
@@ -13,7 +13,7 @@ public class InfixToPrefixTest {
@ParameterizedTest
@MethodSource("provideValidExpressions")
- void testValidExpressions(String infix, String expectedPrefix) throws Exception {
+ void testValidExpressions(String infix, String expectedPrefix) {
assertEquals(expectedPrefix, InfixToPrefix.infix2Prefix(infix));
}