017 Letter Combinations of a Phone Number.py

'''
17. Letter Combinations of a Phone Number
https://leetcode.com/problems/letter-combinations-of-a-phone-number/

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.

A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Example 1:

Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]

Example 2:

Input: digits = ""
Output: []

Example 3:

Input: digits = "2"
Output: ["a","b","c"]

Constraints:
    0 <= digits.length <= 4
    digits[i] is a digit in the range ['2', '9'].
'''
PhoneDigitCharsMap = {
    '2': ['a', 'b', 'c'],
    '3': ['d', 'e', 'f'],
    '4': ['g', 'h', 'i'],
    '5': ['j', 'k', 'l'],
    '6': ['m', 'n', 'o'],
    '7': ['p', 'q', 'r', 's'],
    '8': ['t', 'u', 'v'],
    '9': ['w', 'x', 'y', 'z'],
}

class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        if not digits:
            return []
        results = [[]]
        for digit in digits:
            newResults = []
            for row in results:
                for char in PhoneDigitCharsMap[digit]:
                    newResults.append(row + [char])
            results = newResults
        return [''.join(row) for row in results]