leetcode-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.

Input:Digit string “23”
Output: [“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].
题目大意：根据电话号码键盘上面的数字对应的拼音字母，给一个str型输入，给出所有可能的输出组合。

class Solution(object):
    def letterCombinations(self, digits):
        """ :type digits: str :rtype: List[str] """

        if digits=="":
            return []
        c=[""]
        a={"1":"*","2":"abc","3":"def","4":"ghi","5":"jkl","6":"mno","7":"pqrs","8":"tuv","9":"wxyz","0":" "}
        for i in digits:
            b=[]
            for j in c:
                for k in a[i]:
                    b.append(j+k)
            c=b
        return c

字典保存对应法则。b[] c[]两个list。c保存每一个i对应的输出结果。然后下一个i遍历上一次输出的c里面的结果。输出到b里面，然后c=b。AC。