Solution.java

public class Solution {
    public String numberToWords(int num) {
        if (num==0) {
            return "Zero";
        }
        Map<String, String> map=new HashMap<String, String>();
        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");
        String str=String.valueOf(num);
        int length=str.length();
        List<String> list=new ArrayList<String>();
        while (str.length()>3) {
            list.add(str.substring(length-3,length));
            str=str.substring(0,length-3);
            length=str.length();
        }
        list.add(str);
        System.out.println("list.size()="+list.size());
        List<String> ansList=new ArrayList<String>();
        for (int i = 0; i < list.size(); i++) {
            ansList.add(f(list.get(i),map));
        }
        String ans="";
        if (ansList.size()!=0) {
            ans=ansList.get(0);
            ansList.remove(0);
        }
        if (ansList.size()!=0) {
            if (ansList.get(0).length()!=0) {
                if (ans.length()==0) {
                    ans=ansList.get(0)+" Thousand";
                }else {
                    ans=ansList.get(0)+" Thousand "+ans;
                }
            }
            ansList.remove(0);
        }
        if (ansList.size()!=0) {
            if (ansList.get(0).length()!=0) {
                if (ans.length()==0) {
                    ans=ansList.get(0)+" Million";
                }else {
                    ans=ansList.get(0)+" Million "+ans;
                }
            }
            ansList.remove(0);
        }
        if (ansList.size()!=0) {
            if (ansList.get(0).length()!=0){
                if (ans.length()==0) {
                    ans=ansList.get(0)+" Billion";
                }else {
                    ans=ansList.get(0)+" Billion "+ans;
                }
            }
            ansList.remove(0);
        }
        return ans;
    }
    public String f(String str,Map<String, String> map){
        StringBuffer sb=new StringBuffer();
        int i=Integer.parseInt(str);
        if (i>=100) {
            sb.append(map.get(""+i/100));
            sb.append(" Hundred");
        }
        i=i%100;
        if (i!=0) {
            if (sb.length()!=0) {
                sb.append(" ");
            }
            if (i%100<20) {
                sb.append(map.get(""+i));
            }else {
                sb.append(map.get(""+(i-i%10)));
                if (i%10!=0) {
                    sb.append(" ");
                    sb.append(map.get(""+i%10));
                }
            }
        }
        return sb.toString();
    }
}