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"]]`