leetcode@ [273] Integer to English Words (String & Math)

https://leetcode.com/problems/integer-to-english-words/

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[] c = {"Billion", "Million", "Thousand", ""};
    private final String[] Tens = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    private final String[] Basics = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};

    private String getField(int idx, int size) {
        if(size == 4) {
            if(idx == 0) return "Billion";
            else if(idx == 1) return "Million";
            else if(idx == 2) return "Thousand";
            else if(idx == 3) return "";
        }
        else if(size == 3) {
            if(idx == 0) return "Million";
            else if(idx == 1) return "Thousand";
            else if(idx == 2) return "";
        }
        else if(size == 2) {
            if(idx == 0) return "Thousand";
            else if(idx == 1) return "";
        }
        return "";
    }

    private LinkedList<String> read(int n) {

        LinkedList<String> rs = new LinkedList<String> ();

        int hundred = (n % 1000) / 100;
        int ten = (n % 100) / 10;
        int unit = n % 10;

        if(hundred > 0) {
            rs.addLast(Basics[hundred]);
            rs.addLast("Hundred");
        }
        if(ten > 0) {
            if(ten == 1) {
                int combo = ten * 10 + unit;
                rs.addLast(Basics[combo]);
                return rs;
            } else {
                rs.addLast(Tens[ten]);
            }
        }
        if(unit > 0) {
            rs.addLast(Basics[unit]);
        }
        return rs;
    }

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

        LinkedList<Integer> d = new LinkedList<Integer> ();
        while(num > 0) {
            d.addFirst(num % 1000);
            num/=1000;
        }

        LinkedList<String> ls = new LinkedList<String> ();
        for(int i=0; i<d.size(); ++i) {
            if(d.get(i) == 0) continue;

            LinkedList<String> r = read(d.get(i));
            String field = getField(i, d.size());
            if(i == d.size() - 1) ls.addAll(r);
            else {
                ls.addAll(r);
                ls.addLast(field);
            }
        }
        StringBuffer rs = new StringBuffer();
        for(int i=0; i<ls.size() - 1; ++i) {
            rs.append(ls.get(i));
            rs.append(" ");
        }
        rs.append(ls.peekLast());
        return rs.toString().trim();
    }
}