Integer to English Words, Java

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

//Integer to English Words

/*
Input: 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

Input: 1234567891
Output: "One Billion Two Hundred Thirty Four Million Five Hundred Sixty Seven Thousand Eight Hundred Ninety One"
*/

class Rextester
{

private static final String[] belowTen = new String[]{"","one","two","three","four","five","six","seven","eight","nine"};
    private static final String[] belowTwenty = new String[]{"ten","eleven","twelve","thirteen","fourteen","fifteen","sixteen","seventeen","eighteen","nineteen"};
    private static final String[] belowHundred = new String[]{"","ten","twenty","Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};

public static void main(String args[])
    {
        System.out.println(numberToWords(1234567));
    }

public static String numberToWords(int num) {
        if(num==0)
            return "Zero";
        return helper(num);
    }

public static String helper(int num){
        String out  = new String();
        if (num<10) out = belowTen[num];
        else if (num<20) out = belowTwenty[num-10];
        else if (num<100) out = belowHundred[num/10]+" "+helper(num%10);
        else if (num<1000) out = helper(num/100) + " hundred "+ helper(num%100);
        else if (num<1000000) out = helper(num/1000) + " thousand "+helper(num%1000);
        else if (num<1000000000) out = helper(num/1000000) + " million "+ helper(num%1000000);
        else out = helper(num/1000000000) + " billion "+helper(num%1000000000);
        return out.trim();
    }

}

[ + ] Show input
Compilation time: 0.73 sec, absolute running time: 0.13 sec, cpu time: 0.11 sec, memory peak: 18 Mb, absolute service time: 0,88 sec