*的字母组合

问题：

# 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"]

方法：回溯算法

# leetcode submit region begin(Prohibit modification and deletion)
class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        if not digits: return []
        phone = {'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 backtrack(combination, nextdigit):
            if len(nextdigit) == 0:
                res.append(combination)
                return
            for i in phone[nextdigit[0]]:
                backtrack(combination + i, nextdigit[1:])
        res = []
        backtrack('', digits)
        return res
# leetcode submit region end(Prohibit modification and deletion)