17.letter-combinations-of-a-phone-number.py

#
# @lc app=leetcode id=17 lang=python3
#
# [17] Letter Combinations of a Phone Number
#
# https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/
#
# algorithms
# Medium (40.98%)
# Total Accepted:    367.9K
# Total Submissions: 897.7K
# Testcase Example:  '"23"'
#
# Given a string containing digits from 2-9 inclusive, return all possible
# letter combinations that the number could represent.
#
# A mapping of digit to letters (just like on the telephone buttons) is given
# below. Note that 1 does not map to any letters.
#
#
#
# Example:
#
#
# Input: "23"
# Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
#
#
# Note:
#
# Although the above answer is in lexicographical order, your answer could be
# in any order you want.
#
#


class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        letterDict = {"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']}

        def letterCombHelper(digits, result):
            if len(digits) == 0:
                results.append(result)
                return
            for letter in letterDict[digits[0]]:
                letterCombHelper(digits[1:], result + letter)

        results = []
        if len(digits) != 0:
            letterCombHelper(digits, "")
        return results