273. Integer to English Words

Hard
找的LC上比较满意的答案了，一开始建立一个 LESS_THAN_20和TENS的String array来装常量。然后分为num < 20, num < 100, num < 1000, num < 1000000, num < 100000000来分类。注意一下 +” Hundred "这些都需要前后留一个空格. 还有为什么两个array里要放一个""元素。是为了index可以配合数据，比如TENS[1]是Ten, TENS[2] = Twenty. 注意下billion是十亿不是一亿哦。

class Solution {
    private 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 final String[] TENS = {"","Ten","Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety"};
    public String numberToWords(int num) {
        if (num == 0){
            return "Zero";
        }
        return helper(num);
    }
    
    private String helper(int num){
        String res = "";
        if (num < 20){
            res = LESS_THAN_20[num];
        } else if (num < 100){
            res = TENS[num / 10] + " " + helper(num % 10);
        } else if (num < 1000){
            res = helper(num / 100) + " Hundred " + helper(num % 100);
        } else if (num < 1000000){
            res = helper(num / 1000) + " Thousand " + helper(num % 1000);
        } else if (num < 1000000000){
            res = helper(num / 1000000) + " Million " + helper(num % 1000000);
        } else {
            res = helper(num / 1000000000) + " Billion " + helper(num % 1000000000);
        }
        return res.trim();
    }
}