Number to Words

/*
 * Problem : Write a number in English words.
 * Input   : One integer from 0 to 999999.
 * Output  : The number in lower-case English words.
 * Approach: Handle groups of three digits.
 * Time    : O(1)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    static final String[] ONES = {"", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"};
    static final String[] TENS = {"", "", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"};

    // This method receives a number from 1 to 999 and returns it in words. Example: 345 -> "three hundred forty-five".
    static String underThousand(int number) {
        StringBuilder words = new StringBuilder();

        // Step 1: Write the hundreds. Example: 345 / 100 = 3 -> "three hundred".
        if (number >= 100) {
            words.append(ONES[number / 100]).append(" hundred");
            number = number % 100;
        }

        // Step 2: Write the rest (from 1 to 99). Add a space if something is written already.
        if (number > 0) {
            if (words.length() > 0) {
                words.append(" ");
            }
            if (number < 20) {
                // Step 2a: Numbers below 20 have their own words. Example: 14 -> "fourteen".
                words.append(ONES[number]);
            } else {
                // Step 2b: Write the tens word. Example: 45 -> "forty". Add the ones with a hyphen. Example: "-five".
                words.append(TENS[number / 10]);
                if (number % 10 > 0) {
                    words.append("-").append(ONES[number % 10]);
                }
            }
        }
        return words.toString();
    }

    public static void main(String[] args) {
        int number = new Scanner(System.in).nextInt();

        // Step 3: Zero is a special case. The code below would write nothing.
        if (number == 0) {
            System.out.println("zero");
            return;
        }

        // Step 4: Split the number into thousands and the rest. Example: 123456 -> 123 and 456.
        int thousands = number / 1000;
        int rest = number % 1000;
        StringBuilder result = new StringBuilder();

        // Step 5: Write the thousands part, then the word "thousand".
        if (thousands > 0) {
            result.append(underThousand(thousands)).append(" thousand");
        }

        // Step 6: Write the rest. Add a space if something is written already.
        if (rest > 0) {
            if (result.length() > 0) {
                result.append(" ");
            }
            result.append(underThousand(rest));
        }

        System.out.println(result);
    }
}