leetcode 17 电话号码的字母组合

**LeetCode17:电话号码的字母组合**

### 题目描述给定一个仅包含数字的字符串 `S` 和一个长度为 `4` 的数组 `M`，其中 `M[i]` 是一个大小为 `3` 的字符集合。返回所有可能组合的字母串。

每个字母串都由四个字母组成，每个字母都是从 `M[i]` 中选择的。注意，你不能重复使用相同的字母。

### 示例示例1:

输入: `S = "23", M = ["2","3","4"]`

输出: `[["a","b","c"],["a","d","e"],["a","f","g"],["b","a","c"],["b","d","e"],["b","f","g"],["c","a","d"],["c","b","e"],["c","f","g"]]`

示例2:

输入: `S = "23", M = ["2","3","4"]`

输出: `[["a","b","c"],["a","d","e"],["a","f","g"],["b","a","c"],["b","d","e"],["b","f","g"],["c","a","d"],["c","b","e"],["c","f","g"]]`

示例3:

输入: `S = "23", M = ["2","3","4"]`

输出: `[["a","b","c"],["a","d","e"],["a","f","g"],["b","a","c"],["b","d","e"],["b","f","g"],["c","a","d"],["c","b","e"],["c","f","g"]]`

### 解决方案#### 思路我们可以使用回溯法来解决这个问题。回溯法是一种用于求解满足某些条件的所有可能解的算法。

首先，我们需要定义一个函数 `backtrack` 来实现回溯法。这个函数将接受当前处理的数字和所选字母的集合作为参数。

其次，我们需要定义一个函数 `get_combinations` 来获取所有可能的组合。这个函数将使用回溯法来实现。

最后，我们需要定义一个函数 `letterCombinations` 来返回所有可能的组合。

####代码

class Solution:
 def letterCombinations(self, S: str) -> List[str]:
 if not S:
 return []

 # 定义一个字典来存储数字和对应的字母集合 phone_map = {
 "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"]
 }

 # 定义一个函数来实现回溯法 def backtrack(combination, next_digits):
 if len(next_digits) ==0:
 output.append(combination)
 else:
 for letter in phone_map[next_digits[0]]:
 backtrack(combination + letter, next_digits[1:])

 # 定义一个函数来获取所有可能的组合 def get_combinations():
 output = []
 backtrack("", S)
 return output # 返回所有可能的组合 return get_combinations()

#### 注释* `phone_map` 是一个字典，用于存储数字和对应的字母集合。
* `backtrack` 是一个函数，用于实现回溯法。它接受当前处理的数字和所选字母的集合作为参数。
* `get_combinations` 是一个函数，用于获取所有可能的组合。它使用回溯法来实现。
* `letterCombinations` 是一个函数，用于返回所有可能的组合。

#### 示例示例1:

输入: `S = "23", M = ["2","3","4"]`

输出: `[["a","b","c"],["a","d","e"],["a","f","g"],["b","a","c"],["b","d","e"],["b","f","g"],["c","a","d"],["c","b","e"],["c","f","g"]]`

示例2:

输入: `S = "23", M = ["2","3","4"]`

输出: `[["a","b","c"],["a","d","e"],["a","f","g"],["b","a","c"],["b","d","e"],["b","f","g"],["c","a","d"],["c","b","e"],["c","f","g"]]`

示例3:

输入: `S = "23", M = ["2","3","4"]`

输出: `[["a","b","c"],["a","d","e"],["a","f","g"],["b","a","c"],["b","d","e"],["b","f","g"],["c","a","d"],["c","b","e"],["c","f","g"]]`