273. Integer to English Words.java

273. Integer to English Words
// https://leetcode.com/problems/integer-to-english-words/?tab=Description
LC273把数字转换成英语。然后还是要  run test cases
123 -> "One Hundred Twenty Three"
12345 -> "Twelve Thousand Three Hundred Forty Five"
1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

Hint:
Did you see a pattern in dividing the number into chunk of words? For example, 123 and 123000.
Group the number by thousands (3 digits). You can write a helper function that takes a number less than 1000 and convert just that chunk to words.
There are many edge cases. What are some good test cases? Does your code work with input such as 0? Or 1000010? (middle chunk is zero and should not be printed out)

Test:
0
1000
1000010

注意: i++的位置；最后结果要trim().

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 = 0;
    String res = "";
    while (num > 0) {
        if (num % 1000 != 0) // 3 digits a group
    	    res = helper(num % 1000) +THOUSANDS[i] + " " + res; 
    	num /= 1000;
    	i++; // cannot put into THOUSANDS[i++] !!! e.x. 1000
    }
    return res.trim(); // important
}
private String helper(int num) {
    if (num == 0)    return ""; // necessary! 50868
    else if (num < 20)    return LESS_THAN_20[num] + " "; // 1 - 19
    else if (num < 100)    return TENS[num / 10] + " " + helper(num % 10); // 20,30,40,50,60,70,80,90
    else    return LESS_THAN_20[num / 100] + " Hundred " + helper(num % 100); // > 100
}