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.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Solution1

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        amap = {"2":"abc", "3":"def", "4":"ghi", "5":"jkl", "6":"mno", "7":"pqrs", "8":"tuv", "9":"wxyz"}
        if not digits:
            return []
        return [item for item in reduce(self.combinate, [amap[i] for i in digits])]
        
    def combinate(self, *args):
        return reduce(self.join, args)

    def join(self, astr, bstr):
        for i in astr:
            for j in bstr:
                yield i+j

Solution2

更加簡(jiǎn)潔

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, [''])

思考：

• 列表生成可以容納多個(gè)循環(huán)體
• reduce的功能極其強(qiáng)大，使用之前要遞歸地考慮清楚
• 列表操作1

a = ['']
b = ['123']
print [i + j for i in a for j in b]    # ['123']
print len(a)  # 1