Alphabetical.java

package com.thealgorithms.strings;

import java.util.Locale;

/**
 * Utility class for checking whether a string's characters are in non-decreasing
 * lexicographical order based on Unicode code points (case-insensitive).
 * <p>
 * This does NOT implement language-aware alphabetical ordering (collation rules).
 * It simply compares lowercase Unicode character values.
 * <p>
 * Non-letter characters are not allowed and will cause the check to fail.
 * <p>
 * Reference:
 * <a href="https://en.wikipedia.org/wiki/Alphabetical_order">Wikipedia: Alphabetical order</a>
 */
public final class Alphabetical {

 private Alphabetical() {
 }

 /**
 * Checks whether the characters in the given string are in non-decreasing
 * lexicographical order (case-insensitive).
 * <p>
 * Rules:
 * <ul>
 * <li>String must not be null or blank</li>
 * <li>All characters must be letters</li>
 * <li>Comparison is based on lowercase Unicode values</li>
 * <li>Order must be non-decreasing (equal or increasing allowed)</li>
 * </ul>
 *
 * @param s input string
 * @return {@code true} if characters are in non-decreasing order, otherwise {@code false}
 */
 public static boolean isAlphabetical(String s) {
 if (s == null || s.isBlank()) {
 return false;
 }

 String normalized = s.toLowerCase(Locale.ROOT);

 if (!Character.isLetter(normalized.charAt(0))) {
 return false;
 }

 for (int i = 1; i < normalized.length(); i++) {
 char prev = normalized.charAt(i - 1);
 char curr = normalized.charAt(i);

 if (!Character.isLetter(curr) || prev > curr) {
 return false;
 }
 }
 return true;
 }
}