LeetCode-Integer to English Words

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 231 - 1.

Example 1:

Input: 123
Output: "One Hundred Twenty Three"
Example 2:

Input: 12345
Output: "Twelve Thousand Three Hundred Forty Five"
Example 3:

Input: 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"
Example 4:

Input: 1234567891
Output: "One Billion Two Hundred Thirty Four Million Five Hundred Sixty Seven Thousand Eight Hundred Ninety One"

分成了三种状态。
0-19

20,30,40,...90

thousand, million, billion

所以，给一个三位数，如何转换成英语，这个是核心功能。
于是就用几个数组。

注意的地方是：

else if (num < 100) {
     return TENS[num / 10] + " " + helper(num % 10); } 

必须得用 helper(num % 10)
为什么不能直接 LESS_THAN_20[num % 10] + " "
因为如果是0，不用加空格。你这里无法判断。于是交给helper

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"};
    private final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"};
    
    
    public String numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }
        String ret = "";
        int i = 0;
        while (num > 0) {
            if (num % 1000 != 0) {
                ret = helper(num % 1000) + THOUSANDS[i] + " " + ret;
            }
            num = num / 1000;
            i++;
        }
        
        return ret.trim();
    }
    
    private String helper(int num) {
        if (num == 0) {
            return "";
        }
        else if (num < 20) {
            return LESS_THAN_20[num] + " ";
        }
        else if (num < 100) {
            return TENS[num / 10] + " " + helper(num % 10);
        }
        else {
            return LESS_THAN_20[num / 100] + " Hundred " + helper(num % 100);
        }
    }
}