017._letter_combinations_of_a_phone_number.md

###17. Letter Combinations of a Phone Number

题目:

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

难度:

Medium

思路：

• hash table一个，用来对应digit -> letter
• s用来记录结果，每次从digits里面去一个，然后寻找其可能的char，加到s中，digits长度减小
• digits长度为0时候，把它加入结果

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        if digits == "": return []
        self.res = []
        def letterGenerate(digits, curStr):
            table = {"2":"abc","3":"def","4":"ghi","5":"jkl",
                    "6":"mno","7":"pqrs","8":"tuv","9":"wxyz"}
            if digits == "":
                self.res += [curStr]
            else:
                for char in table[digits[0]]:
                    letterGenerate(digits[1:],curStr + char)

        letterGenerate(digits, "")
        return self.res

这个很像permutation的变体。