17. Letter Combinations of a Phone Number[M]

https://leetcode.com/problems/letter-combinations-of-a-phone-number/

Description

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.

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Example:

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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.

Solution

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class Solution:
    def __init__(self):
        self.map = {'1':"", '2':"abc", '3':"def", '4':"ghi", '5':"jkl", 
                    '6':"mno", '7':"pqrs", '8':"tuv", '9':"wxyz"}
        self.res = []
    def letterCombinations(self, digits: str) -> List[str]:
        if not digits:
            return None
        tmp = ""
        self.combo(digits, tmp)
        return self.res
        
    def combo(self, digits, tmp):
        if not digits:
            self.res.append(tmp)
        else:
            for ch in self.map[digits[0]]:
                self.combo(digits[1:],tmp + ch)