class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        if not digits: return []
        d = {
            '2': 'abc',
            '3': 'def',
            '4': 'ghi',
            '5': 'jkl',
            '6': 'mno',
            '7': 'pqrs',
            '8': 'tuv',
            '9': 'wxyz',
        }

        # 寻求可行解 回溯法
        def backtrack(res, index, l, path):
            if (len(path) == l):
                res.apPEnd(''.join(path))
            else:
                key = digits[index]
                for s in d[key]:
                    path.append(s)
                    backtrack(res, index + 1, l, path)
                    path.pop() # 满足条件，开始回溯

        l = len(digits)
        res = []
        path = []
        backtrack(res, 0, l, path)
        return res