Java Solution with one lookup table


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    This solution uses just one lookup table and easy to understand:

    public class Solution {
        public String numberToWords(int num) {
            HashMap<Integer, String> map = new HashMap<Integer, String> ();
            map.put(0, "Zero");
            map.put(1, "One");
            map.put(2, "Two");
            map.put(3, "Three");
            map.put(4, "Four");
            map.put(5, "Five");
            map.put(6, "Six");
            map.put(7, "Seven");
            map.put(8, "Eight");
            map.put(9, "Nine");     
            map.put(10, "Ten");
            map.put(11, "Eleven");
            map.put(12, "Twelve");
            map.put(13, "Thirteen");
            map.put(14, "Fourteen");
            map.put(15, "Fifteen");
            map.put(16, "Sixteen");
            map.put(17, "Seventeen");
            map.put(18, "Eighteen");
            map.put(19, "Nineteen");
            map.put(20, "Twenty");
            map.put(30, "Thirty");
            map.put(40, "Forty");
            map.put(50, "Fifty");
            map.put(60, "Sixty");
            map.put(70, "Seventy");
            map.put(80, "Eighty");
            map.put(90, "Ninety");
            map.put(100, "Hundred");
            map.put(1000, "Thousand");
            map.put(1000000, "Million");
            map.put(1000000000, "Billion");
            
            int[] steps= new int[]{1000000000, 1000000, 1000, 1};
            StringBuilder sb = new StringBuilder();
            for(int step: steps) {
                if (num/step>0)  {
                    sb.append(helper(num/step, map));
                    if (step>1) {
                        sb.append(map.get(step)).append(" ");
                    }
                }
                num = num%step;
            }
            String result = sb.toString().trim();
            if (result.length()==0) {
                return "Zero";
            }
            else {
                return result;
            }
        }
        String helper(int n, Map<Integer,String>map) {
            StringBuilder sb = new StringBuilder();
            if (n/100>0) {
                sb.append(map.get(n/100)).append(" ");
                sb.append(map.get(100)).append(" ");
                n = n%100;
            }
            if (n/20>0){
                sb.append(map.get(n-n%10)).append(" ");
                n = n%10;
            }
            if (n>0) {
                sb.append(map.get(n)).append(" ");
            }
            return sb.toString();
        }
    }

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