Integer to English Words

题目：

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 2^31 - 1.

For example,
123 -> "One Hundred Twenty Three"
12345 -> "Twelve Thousand Three Hundred Forty Five"
1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

分析：英语中只有十（ten），百（hundred），千（thousand），百万（million），十亿（billion），万亿（trillion），因为输入的数最大为2^31 - 1，故这里用不着万亿(trillion)，其中1 trillion = 1000 billion = 10^6 million = 10^9 thousand = 10^12. 另外注意英语数字中小于20的单词都是不唯一不能组合的。

思路：从数字的低位开始，结合辅助数组，每三位(1000)处理一次，代码如下：

public class Solution {
    String[] lessThan20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    String[] tens = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    String hundred = "Hundred";
    String[] thousands = {"", "Thousand", "Million", "Billion"};
    public 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(); //删除尾部的空格
    }
    private String helper(int i) {
        if (i == 0) {
            return "";
        } else if (i < 20) {
            return lessThan20[i] + " ";
        } else if (i < 100) {
            return tens[i/10] + " " + helper(i%10);
        } else {
            return helper(i/100) + hundred  + " " + helper(i%100);
        }
    }
}