example1.py

#!/usr/bin/python

from collections import deque


def combinations(digits):
    keys = {
        "2": "abc",
        "3": "def",
        "4": "ghi",
        "5": "jkl",
        "6": "mno",
        "7": "pqrs",
        "8": "tuv",
        "9": "wxyz",
    }

    queue = deque()

    if digits:
        queue.append("")

    while queue and len(queue[0]) < len(digits):
        current_combination = queue.popleft()
        next_digit = digits[len(current_combination)]
        letters = keys[next_digit]

        for letter in letters:
            queue.append(current_combination + letter)

    return list(queue)


print(combinations("23")) # ['ad', 'ae', 'af', 'bd', 'be', 'bf', 'cd', 'ce', 'cf']
print(combinations("")) # []
print(combinations("2")) # ['a', 'b', 'c']