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

For example,

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123 -> "One Hundred Twenty Three"
12345 -> "Twelve Thousand Three Hundred Forty Five"
1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"
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public class Solution {
        final String[] LESS_THAN_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        final String[] TENS = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        final String[] THOUSANDS = {"Billion", "Million", "Thousand", ""};
        final int[] radix = new int[]{1000000000, 1000000, 1000, 1};

    public String numberToWords(int num) {
        if (num == 0) return "Zero";
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < radix.length; i++) {
            if (num / radix[i] == 0) continue;
            sb.append(trans(num / radix[i])).append(THOUSANDS[i]).append(' ');
            num %= radix[i];
        }
        return sb.toString().trim();
    }

    private String trans(int num) {
        if (num == 0) return "";
        if (num < 20) return LESS_THAN_20[num] + " ";
        if (num < 100) return TENS[num / 10] + " " + trans(num % 10);
        return LESS_THAN_20[num / 100] + " Hundred " + trans(num % 100);
    }
}

Inefficient using String catenation solution

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public class Solution {
    private static final String[] TWENTY = new String[]{"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten","Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private static final String[] TENS = new String[]{"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    private static final String[] THOUSANDS = new String[]{"", "Thousand", "Million", "Billion"};
        
    public String numberToWords(int num) {
        if (num == 0)
            return "Zero";
            
        int i = 0;
        String result = "";
        
        while (num > 0) {
            if (num % 1000 != 0) {
                result = getString(num % 1000) + THOUSANDS[i] + " " + result;
            }
            num /= 1000;
            i++;
        }
        return result.trim();
    }
    
    private String getString(int num) {
        if (num == 0) {
            return "";
        } else if (num < 20) {
            return TWENTY[num] + " ";
        } else if (num < 100) {
            return TENS[num / 10] + " " + getString(num % 10);
        } else {
            return TWENTY[num / 100] + " Hundred " + getString(num % 100);
        }
    }
}