# Letter Combinations of a Phone Number(dfs)

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.

![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Telephone-keypad2.svg/200px-Telephone-keypad2.svg.png)

**Example:**

```
Input: 
"23"

Output:
 ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
```

分析：

dfs,lettermap可以用dict也可以array

```
class Solution:
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        if not digits:
            return []
        # lettermap = dict([('0',""),('1',""),('2',"abc"),('3',"def"),('4',"ghi"),('5',"jkl"),('6',"mno"),('7',"pqrs"),('8',"tuv"),('9',"wxyz")])
        lettermap = ["","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"]
        ret=[]
        self.dfs(digits,ret,"",0,lettermap)
        return ret

    def dfs(self,digits,ret,path,pos,lettermap):
        if pos == len(digits):
            ret.append(path)
            return 
        for c in lettermap[int(digits[pos])]:
            self.dfs(digits,ret,path+c,pos+1,lettermap)
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://nataliekung.gitbook.io/ladder_code/facebook/letter-combinations-of-a-phone-numberdfs.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.