Integer to English Words

题目地址：
https://leetcode.com/problems/integer-to-english-words/#/description

题目：
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"

解题思路：
这道题首先要建立3个array-> LESS_THAN_20, TENS, THOUSANDS。然后先对thousand的加进去。helper的function需要讨论是否为0，是否小于20，是否小于100，然后就是最后的。

代码：

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"};
private final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"};

public String numberToWords(int num) {
    if(num == 0){
        return "Zero";
    }
    int i_thousand = 0;
    String words = "";
    while(num > 0){
        if(num % 1000 != 0){
            words  = helper(num % 1000) + THOUSANDS[i_thousand] + " " + words;
        }
        i_thousand += 1;
        num /= 1000;
    }
    return words.trim();
}

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