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"

Recursion

思路

三位一算，比如1234567，先算567，得到Five Hundred Sixty Seven，再算234，得到Two Hundred Thirty Four Thousand，算出的结果后面加上thousand，再算1,加上billion

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";
    String word = "";
    int i = 0;
    while(num > 0){
        if(num % 1000 != 0){
            word = helper(num % 1000) + THOUSANDS[i] + " " + word;
        }
        num /= 1000;
        i++;
    }
    return word.trim();
}

private String helper(int num){
    if(num == 0) return "";
    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);
    }
}