leetcodeday17 电话号码的字母组合

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

示例 1：

输入：digits = “23”
输出：[“ad”,”ae”,”af”,”bd”,”be”,”bf”,”cd”,”ce”,”cf”]
示例 2：

输入：digits = “”
输出：[]
示例 3：

输入：digits = “2”
输出：[“a”,”b”,”c”]

提示：

• 0 <= digits.length <= 4
• digits[i] 是范围 ['2', '9'] 的一个数字

思路：遍历递归求解

# @lc code=start
class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        if digits=="": 
            return []
        n=len(digits)
        sets={"2":["a","b","c"],"3":["d","e","f"],"4":["g","h","i"],"5":["j","k","l"],"6":["m","n","o"],"7":["p","q","r","s"],"8":["t","u","v"],"9":["w","x","y","z"]}
        def upsort(strs): 
            if len(strs)==1: 
                return sets[strs]
    
            result = upsort(strs[:-1])
            rev=[]
            for i in range(len(result)):
                 for j in sets[strs[-1]]:
                    rev.append(result[i]+j)
            return rev
        return upsort(digits)