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

python3 复制代码
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