Leetcode Integer to English Words problem solution

class Solution:
    def numberToWords(self, num: int) -> str:
        self.lessThan20 = ["", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"]
        self.tens = ["", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"]
        self.thousands = ["", "Thousand", "Million", "Billion"];
        if num == 0 :
            return "Zero"
        res = ''
        i = 0
        while num>0:
            if num%1000 != 0:
                res = self.helper(num%1000) + self.thousands[i] + " " + res
            num //= 1000
            i += 1
        return res.strip()
    
    def helper(self, num):
        if num == 0:
            return ""
        if num < 20:
            return self.lessThan20[num] + " "
        if num < 100:
            return self.tens[num//10] + " " + self.helper(num%10)
        if num < 1000:
            return self.lessThan20[num//100] + " Hundred " + self.helper(num%100)

class Solution {
    static HashMap<Integer,String> hm;
    public static void fill() {
        hm = new HashMap<Integer, String>();
        hm.put(1_000_000_000,"Billion");
        hm.put(1_000_000,"Million");
        hm.put(1_000,"Thousand");
        hm.put(100, "Hundred");
    }
    static String ten[] = {"","One","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Eleven","Twelve",
            "Thirteen","Fourteen","Fifteen","Sixteen","Seventeen","Eighteen","Nineteen"};
    static String two[] = {"","","Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety"};
    public static String numberToWords(int n) {
        if(n<1)
            return "Zero";
        fill();
        int base = 1_000_000_000;
        StringBuilder sb = new StringBuilder();
        while(n>0&&base>1) {
            if(n>=base) {
                int val = n/base;
                sb.append(get(val));
                sb.append(hm.get(base)+" ");
                n%=base;
            }
            base/=1000;
        }
        sb.append(get(n));
        sb.deleteCharAt(sb.length()-1);
        return sb.toString();
    }
    public static String get(int n) {
        StringBuilder sb = new StringBuilder();
        while(n>0) {
            if(n>=100) {
                int val=n/100;
                sb.append(ten[val]+" ");
                sb.append(hm.get(100)+" ");
                n%=100;
            }
            else if(n>=20) {
                int val = n/10;
                sb.append(two[val]+" ");
                n%=10;
            }
            else if(n<20) {
                sb.append(ten[n]+" ");
                n=0;
            }
        }
        return sb.toString();
    }
}

class Solution {
public:
    string hundredStr(int num){
        vector<string> arr1={"","One","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten",
        "Eleven","Twelve","Thirteen","Fourteen","Fifteen","Sixteen","Seventeen","Eighteen","Nineteen"};
        vector<string> arr2={"","","Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety"};
        string ret;
        ret=num%100<20?arr1[num%100]:arr2[(num%100)/10]+(num%10?" "+arr1[num%10]:"");
        if(num>99)ret=arr1[num/100]+" Hundred"+(num%100?" "+ret:"");
        return ret;
    }
    string numberToWords(int num) {
        string ret;
        vector<string> strarr={"Thousand","Million","Billion"};
        ret=hundredStr(num%1000);
        for(int i=0;i<3;i++){
            num/=1000;
            ret=num%1000?hundredStr(num%1000)+" "+strarr[i]+" "+ ret:ret;
        }
        while(ret.back()==' ')ret.pop_back();
        return ret.empty()?"Zero":ret;
    }
};