Leetcode 273. Integer to English Words

Problem

Convert a non-negative integer num to its English words representation.

Algorithm

Three consecutive digit make one group. Simulation processing will suffice and specifically deal the number less than 20.

Code

class Solution:
    def numberToWords(self, num: int) -> str:

        def units(num):
            if    1 == num: return "One"
            elif  2 == num: return "Two"
            elif  3 == num: return "Three"
            elif  4 == num: return "Four"
            elif  5 == num: return "Five"
            elif  6 == num: return "Six"
            elif  7 == num: return "Seven"
            elif  8 == num: return "Eight"
            elif  9 == num: return "Nine"
            elif 10 == num: return "Ten"
            elif 11 == num: return "Eleven"
            elif 12 == num: return "Twelve"
            elif 13 == num: return "Thirteen"
            elif 14 == num: return "Fourteen"
            elif 15 == num: return "Fifteen"
            elif 16 == num: return "Sixteen"
            elif 17 == num: return "Seventeen"
            elif 18 == num: return "Eighteen"
            elif 19 == num: return "Nineteen"
            return ""

        def tens(num):
            if   2 == num: return "Twenty"
            elif 3 == num: return "Thirty"
            elif 4 == num: return "Forty"
            elif 5 == num: return "Fifty"
            elif 6 == num: return "Sixty"
            elif 7 == num: return "Seventy"
            elif 8 == num: return "Eighty"
            elif 9 == num: return "Ninety"
            return ""

        def two_digit(num):
            if num < 20: 
                return units(num)
            else:
                ans = tens(num // 10)
                num = num - num // 10 * 10
                if num > 0:
                    ans += " " + units(num - num // 10 * 10)
                return ans

        def three_digit(num):
            ans = ""
            if num >= 100:
                ans += units(num // 100) + " Hundred"
                num = num - num // 100 * 100
            if num > 0:
                if ans != "": ans += " "
                ans += two_digit(num)
            return ans
        
        if 0 == num:
            return "Zero"

        ans = ""
        if num >= 1000000000: 
            ans += three_digit(num // 1000000000) + " Billion"
            num = num - num // 1000000000 * 1000000000
        if num >= 1000000:
            if ans != "": ans += " "
            ans += three_digit(num // 1000000) + " Million"
            num = num - num // 1000000 * 1000000
        if num >= 1000: 
            if ans != "": ans += " "
            ans += three_digit(num // 1000) + " Thousand"
            num = num - num // 1000 * 1000
        if num > 0:
            if ans != "": ans += " "
            ans += three_digit(num)

        return ans