273. Integer to English Words

// 看看人家的转换int为String吧，真的很不错啊！主要是在1000以内的数自由转换的时候我没做好
    private static final String[] lessThan20 = { "", "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" };
    /*
     * 273. Integer to English Words 
     * 1.14 by Mingyang
     * 这个题目自己动手做的时候一团mess，所以我们看看这个网上比较精炼的代码
     * 这个题目其实很简单，从最低为开始，每三位得出一个读书，加上计量单位，依次往后
     * 那么重点在于三位数怎么读，怎么分类：
     * 1.小于20，自然直接读出来了
     * 2.小于100，需要加几十的读音，加剩下的数（不能再用小于20数组，因为空格不对）
     * 3.大于100的先加hundred。再来数字余下的
     */
    public static String numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }
        String result = "";
        int i = 0;
        while (num > 0) {
            if (num % 1000 != 0) {
                result = helper(num % 1000) + thousands[i] + " " + result;
            }
            num /= 1000;
            i++;
        }
        return result.trim();
    }
    private static String helper(int num) {
        if (num == 0) {
            return "";
        } else if (num < 20) {
            return lessThan20[num] + " ";
        } else if (num < 100) {
//这里我们把helper改为lessThan20，因为我们这里有个空格fifty thousand只有一个空格，
//而lessThan20[num] + " "（多一个空格）
            return tens[num / 10] + " " + helper(num % 10);
        } else {
            return lessThan20[num / 100] + " Hundred " + helper(num % 100);
        }
    }