整数的英语表示

/*
思路：
1. 使用与上述Java解法相同的思路，但尽可能地使用Java Stream API来处理数据。
*/
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class NumberToWordsStream {
    private static final String[] LESS_THAN_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private static final String[] TENS = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    private static final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"};

    public String numberToWords(int num) {
        if (num == 0) return "Zero";

        int[] parts = new int[4];
        int i = 0;
        while (num > 0) {
            parts[i++] = num % 1000;
            num /= 1000;
        }

        return IntStream.range(0, 4)
                .mapToObj(j -> parts[j] > 0 ? helper(parts[j]) + THOUSANDS[j] : "")
                .filter(s -> !s.isEmpty())
                .collect(Collectors.joining(" "))
                .trim();
    }

    private String helper(int num) {
        if (num == 0)
            return "";
        else if (num < 20)
            return LESS_THAN_20[num] + " ";
        else if (num < 100)
            return TENS[num / 10] + " " + helper(num % 10);
        else
            return LESS_THAN_20[num / 100] + " Hundred " + helper(num % 100);
    }
}