17. Letter Combinations of a Phone Number

Given a digit string, 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.

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My Solution

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        if digits == "":
            return []
        dict_d = {'1': ' ', '2': 'abc', '3': 'def', '4': 'ghi', '5': 'jkl', '6': 'mno', '7': 'pqrs', '8': 'tuv', '9': 'wxyz', '0': '*'}
        result = list(dict_d[digits[0]])
        for i in range(1, len(digits)):
            result = [x + y for x in result for y in dict_d[digits[i]]]
        return result

Reference (转)

1:

class Solution:
    # @return a list of strings, [s1, s2]
    def letterCombinations(self, digits):
        if '' == digits: return []
        kvmaps = {
            '2': 'abc',
            '3': 'def',
            '4': 'ghi',
            '5': 'jkl',
            '6': 'mno',
            '7': 'pqrs',
            '8': 'tuv',
            '9': 'wxyz'
        }
        return reduce(lambda acc, digit: [x + y for x in acc for y in kvmaps[digit]], digits, [''])

2:

class Solution:
    # @param {string} digits
    # @return {string[]}
    def letterCombinations(self, digits):
        mapping = {'2': 'abc', '3': 'def', '4': 'ghi', '5': 'jkl', 
                   '6': 'mno', '7': 'pqrs', '8': 'tuv', '9': 'wxyz'}
        if len(digits) == 0:
            return []
        if len(digits) == 1:
            return list(mapping[digits[0]])
        prev = self.letterCombinations(digits[:-1])
        additional = mapping[digits[-1]]
        return [s + c for s in prev for c in additional]