letter_combination.py

class Solution:
    def letterCombinations(self, digits):
        if not digits:
            return []

        mapping = {
            "2": "abc",
            "3": "def",
            "4": "ghi",
            "5": "jkl",
            "6": "mno",
            "7": "pqrs",
            "8": "tuv",
            "9": "wxyz",
        }

        result = []

        def backtrack(solution, idx):
            if idx == len(digits):
                result.append("".join(solution))
                return

            for letter in mapping.get(digits[idx], ""):
                solution.append(letter)
                backtrack(solution, idx + 1)
                solution.pop()

        backtrack([], 0)
        return result


if __name__ == "__main__":

    solution = Solution()

    digits = "23"
    result = solution.letterCombinations(digits)
    assert result == ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]
    print(result)
    print("Test Case 1 Passed!")