leetcode17 Letter Combinations of a Phone Number

"""
Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.
Example:
Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
"""
"""
本题三次循环，看不懂debug一遍就好了
"""
class Solution1:
    def letterCombinations(self, digits):
        phone = {'2': 'abc', '3': 'def', '4': 'ghi', '5': 'jkl', '6': 'mno', '7': 'pqrs', '8': 'tuv', '9': 'wxyz'}
        digits = 23
        if digits == '':
            return []
        result = ['']
        for digit in digits:
            lst = phone[digit]
            newresult = []
            for x in result:   #!!!保存现有的排列组合
                for y in lst:  #
                    newresult.append(x + y)
            result = newresult
        return result

# phone = {'2': 'abc', '3': 'def', '4': 'ghi', '5': 'jkl', '6': 'mno', '7': 'pqrs', '8': 'tuv', '9': 'wxyz'}
# digits = '23'
# result = ['']
# for digit in digits:
#     lst = phone[digit]
#     newresult = []
#     for x in result:   #!!!保存现有的排列组合
#         for y in lst:  #
#             newresult.append(x + y)
#     result = newresult
#
# print(result)