199. Integer to English Words

Split the number into 3-digit groups from the right. Spell each group (hundreds/tens/ones) and append its scale word — Thousand, Million, Billion.

Group by thousands

English names numbers in 3-digit groups. Peel off num % 1000 at a time; spell that group with a helper (X Hundred Y Z), append the scale word for its position (Thousand / Million / Billion), and prepend to the result. Zero groups are skipped.

helper(n): words for a value < 1000
while num: chunk = helper(num%1000) + scale[i]; prepend; num //= 1000; i++

Worked example

1234567 → One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven.

Complexity

• Time: O(1) (at most 10 digits).
• Space: O(1).

import java.util.ArrayList;
import java.util.List;

class Solution {
    private String[] below20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight",
            "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen",
            "Seventeen", "Eighteen", "Nineteen"};
    private String[] tens = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    private String[] thousands = {"", "Thousand", "Million", "Billion"};

    public String numberToWords(int num) {
        if (num == 0) return "Zero";
        List<String> res = new ArrayList<>();
        int i = 0;
        while (num > 0) {
            if (num % 1000 != 0) {
                List<String> chunk = helper(num % 1000);
                if (!thousands[i].isEmpty()) chunk.add(thousands[i]);
                res.addAll(0, chunk);
            }
            num /= 1000;
            i++;
        }
        return String.join(" ", res);
    }

    private List<String> helper(int n) {
        List<String> r = new ArrayList<>();
        if (n == 0) return r;
        if (n < 20) {
            r.add(below20[n]);
        } else if (n < 100) {
            r.add(tens[n / 10]);
            r.addAll(helper(n % 10));
        } else {
            r.add(below20[n / 100]);
            r.add("Hundred");
            r.addAll(helper(n % 100));
        }
        return r;
    }
}