PalindromeChecker.java

package module4.recursion.palindromes;

/**
 * PalindromeChecker
 * 
 * @author Mae Morella
 */
public class PalindromeChecker {

    /**
     * Detects palindromes using a recursive method. 
     * A palindrome is any string whose letters are the same forward as they are backwards. 
     * Non-alphanumeric characters are ignored; for example, {@code "Madam I'm adam"} is 
     * converted to {@code "madamimadam"}, and considered a valid palindrome.
     * 
     * @param str The string to test
     * @return If the string is a valid palindrome, {@code true}. Otherwise
     *         {@code false}
     */
    public static boolean isPalindrome(String str) {
        String cleanString = str.toLowerCase().replaceAll("[^a-zA-Z0-9]", "");
        return isPalindromeRecursive(cleanString);
    }

    private static boolean isPalindromeRecursive(String str) {
        if (str.length() < 2) return true;

        char first = str.charAt(0);
        char last = str.charAt(str.length() - 1);
        if (first == last) {
            String inner = str.substring(1, str.length() - 1);
            return isPalindromeRecursive(inner);
        } else {
            return false;
        }
    }
}