17. Letter Combinations of a Phone Number

# 递归方法解决
class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        d = {"2":"abc", "3":"def", "4":"ghi", "5":"jkl", "6":"mno", "7":"pqrs", "8":"tuv", "9":"wxyz"}
        # 设置终止条件
        if len(digits) == 0:
            return []
        if len(digits) == 1:
            return list(d[digits[0]])

        prev = self.letterCombinations(digits[:-1])
        addtional = d[digits[-1]]
        
        return [s + c for s in prev for c in addtional]