Solution.py

class Solution:
    def numberToWords(self, num: int) -> str:
        if num == 0:
            return 'Zero'

        lt20 = ['', 'One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Eleven',
                'Twelve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen', 'Seventeen', 'Eighteen', 'Nineteen']
        tens = ['', 'Ten', 'Twenty', 'Thirty', 'Forty',
                'Fifty', 'Sixty', 'Seventy', 'Eighty', 'Ninety']
        thousands = ['Billion', 'Million', 'Thousand', '']

        def transfer(num):
            if num == 0:
                return ''
            if num < 20:
                return lt20[num] + ' '
            if num < 100:
                return tens[num // 10] + ' ' + transfer(num % 10)
            return lt20[num // 100] + ' Hundred ' + transfer(num % 100)

        res = []
        i, j = 1000000000, 0
        while i > 0:
            if num // i != 0:
                res.append(transfer(num // i))
                res.append(thousands[j])
                res.append(' ')
                num %= i
            j += 1
            i //= 1000
        return ''.join(res).strip()