leetcode- Letter Combinations of a Phone Number

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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.

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        if digits=='':
            return []
        digit_to_str={'2':'abc','3':'def','4':'ghi','5':'jkl','6':'mno','7':'pqrs','8':'tuv','9':'wxyz'}
        
        result=['']
        for x in digits:
            tmp=[]
            for y in result:
                for z in digit_to_str[x]:
                    tmp.append(y+z)
            result=tmp
            
        return result

对比优秀解法

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, [''])