Integer to English Words

题意：

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 231 - 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：
三位三位的从后往前数，分别以"","thousand","million","billion"为单位

注意单词不要拼写错误！！！

参考代码：
https://leetcode.com/discuss/55462/my-clean-java-solution-very-easy-to-understand

public class Solution {
private final String[] lessThan20 = {"", "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"};
private final String[] thousands = {"", "Thousand", "Million", "Billion"};

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

while (num > 0) {
if (num % 1000 != 0)
words = helper(num % 1000) + thousands[i] + " " + words;
num /= 1000;
i++;
}

return words.trim();
}

private String helper(int num) {
if (num == 0)
return "";
else if (num < 20)
return lessThan20[num] + " ";
else if (num < 100)
return tens[num / 10] + " " + helper(num % 10);
else
return lessThan20[num / 100] + " Hundred " + helper(num % 100);
}
}