Phone Words

Working through a few problems on LeetCode. I haven’t quite decided what I think of the site, but it’s a fun way to play with simple algorithms. Figured I might as well write up any I find interesting.

First interesting problem:

Given a standard lettered keypad, generate all words from a given phone number.

def letterCombinations(self, digits: str) -> List[str]:
    if not digits:
        return []

    letters = {
        '1': '',
        '2': 'abc',
        '3': 'def',
        '4': 'ghi',
        '5': 'jkl',
        '6': 'mno',
        '7': 'pqrs',
        '8': 'tuv',
        '9': 'wxyz',
        '0': ' ',

    return [
        for product in itertools.product(*[
            letters[digit] for digit in digits

I’m going to be perhaps a little overzealous when it comes to coding that. From the inside out:

  • Loop over the digits with for digit in digits
  • Get the letters for each of those with letters[digit]
  • Generate all combinations of letters with itertools.product (strings are iterable)
  • Join then all back together with ''.join(product)

And that’s all we need. I do love list comprehension in Python. Makes me think I’m writing in a proper language like Lisp/Scheme/Racket. :D (I should write more Racket, it’s been a while).

A few tests to make sure I’m doing something sensible:

class TestSolution(unittest.TestCase):
    def test_1(self):
            ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]

    def test_2(self):

    def test_3(self):
            ["a", "b", "c"]

if __name__ == '__main__':

And there we go:

Ran 3 tests in 0.000s