// 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"

public 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";

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);

}

}

}