整数转英文表示

class Solution:
    def numberToWords(self, num: int) -> str:
       one_to_19 = '''One Two Three Four Five Six Seven Eight Nine Ten Eleven Twelve
               Thirteen Fourteen Fifteen Sixteen Seventeen Eighteen Nineteen'''.split()
       tens = 'Twenty Thirty Forty Fifty Sixty Seventy Eighty Ninety'.split()

       def helper(num):
           if num < 20:
               return one_to_19[num - 1:num]
           elif num < 100:
               return [tens[num // 10 - 2]] + helper(num % 10)
           elif num < 1000:
               return [one_to_19[num // 100 -1]] + ['Hundred'] + helper(num % 100)

           for p,w in enumerate(['Thousand','Million','Billion'],1):
               if num < 1000 ** (p+1):
                   return helper(num//1000**p) + [w] + helper(num % 1000 ** p)

        return ' '.join(helper(num)) or "Zero"