How hard is LeetCode 273. Integer to English Words?

LeetCode 273. Integer to English Words is rated Hard on LeetCode.

What topics does LeetCode 273. Integer to English Words cover?

LeetCode 273. Integer to English Words is tagged Recursion, Math and String on LeetCode.

Integer to English Words — LeetCode 273 Python Solution

HardRecursionMathString

Problem: #273
Pattern: Math and Number Theory
Reading time: 10 min
Source: leetcode.com

The problem

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

Example

Input: num = 123
Output: "One Hundred Twenty Three"

Python solution

Python

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()

Complexity

Measure	Complexity
Time	O(n) or O(1)
Space	O(1) auxiliary

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 273. Integer to English Words is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Math.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 273. Integer to English Words?: LeetCode 273. Integer to English Words is rated Hard on LeetCode.
What topics does LeetCode 273. Integer to English Words cover?: LeetCode 273. Integer to English Words is tagged Recursion, Math and String on LeetCode.