# 273 Integer to English Words

把一個非負整數轉成英文。

Concept:

1234567

要從最小位數開始，每三位一個單位。

567 -> Five Hundred Sixty Seven

下一個三位

234 -> Two Hundred Thirty Four

後面要接 Thousand

-Main

20以下都是獨立的單字，從20以上開始需要組合，

所以用LESS_THAN_20來列舉20以下的數字，用TENS來列舉10,20,30...90。

用一個THOUSANDS陣列來列舉出Thousand, Million, Billion

THOUSANDS的index i每算完三位就++

並用一個helper function來計算千位以下的轉換

-helper(int num)

少於20的，從LESS_THAN_20取出來，

小於100的，取出TENS[num/10]，再將num%10丟給helper

其他，取出LESS_THAN_20[num/100]，接上Hundred，再將num%100丟給helper

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 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 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);
}