273. Integer to English Words

Hard

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

Example 1:

Input: 123
Output: "One Hundred Twenty Three"

Example 2:

Input: 12345
Output: "Twelve Thousand Three Hundred Forty Five"

Example 3:

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

Example 4:

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

Solution:
three digits one group, parse 3 digits.

    public String numberToWords(int num) {
        if(num == 0) return "Zero";
        StringBuilder sb = new StringBuilder();
        int threeDigits = num %1000;
        sb.insert(0,parse3digits(threeDigits));
        String[] thousands = {"Thousand","Million","Billion"};
        int index = 0;
        while(num > 0){
            num = num/1000;
            threeDigits = num %1000;
            if(threeDigits > 0){
                sb.insert(0,parse3digits(threeDigits) +" "+ thousands[index]+" ");
            }
            index++;
        }
        String res = sb.toString().trim();
        return res;
    }
    
    public String parse3digits(int num){
        String[] to19 = {"","One","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Eleven","Twelve","Thirteen","Fourteen","Fifteen","Sixteen","Seventeen","Eighteen","Nineteen"};
        String[] tens = {"","","Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety"
        };
        int firstDigit = num/100;
        StringBuilder sb = new StringBuilder();
        if(firstDigit > 0){
            sb.append(to19[firstDigit] + " Hundred ");
        }
        int lastTwoDigits = num%100;
        
        if(lastTwoDigits >= 20){
            int secondDigit = lastTwoDigits/10;
            
            sb.append(tens[secondDigit] + " ");
            int lastDigit = num%10;
            sb.append(to19[lastDigit]);
        }else{
            sb.append(to19[lastTwoDigits]);
        }
        
        return sb.toString().trim();
    }

Solution2:
The given number is divided in two parts: the part divide 

class Solution {
    public String numberToWords(int num) {
        if(num == 0) return "Zero";
        String res = in2String(num);
        return res.trim();
    }
    
    public String in2String(int num){
        String[] tens =       {"","","Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety"};
        String res = "";
        if(num>=1000000000){
            res = in2String(num/1000000000) + " Billion " + in2String(num%1000000000);
        }else if(num >= 1000000){
            res = in2String(num/1000000) + " Million " + in2String(num%1000000);
        }else if(num >= 1000){
            res = in2String(num/1000) + " Thousand " + in2String(num%1000);
        }else if(num >= 100){
            res = in2String(num/100) + " Hundred " + in2String(num%100);
        }else if(num >= 20){
            int secondDigit = num/10;
            res = tens[secondDigit] + " " + in2String(num%10);
        }else if(num > 0){
            String[] to19 = {"","One","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Eleven","Twelve","Thirteen","Fourteen","Fifteen","Sixteen","Seventeen","Eighteen","Nineteen"};
            res = " "+ to19[num];
        }
        return res.trim();
    }
}