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”].
Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

翻譯
給定一個僅包含數字 2-9 的字串，返回所有它能表示的字母組合。
給出數字到字母的對映如下（與電話按鍵相同）。注意 1 不對應任何字母。

示例:
輸入：“23”
輸出：[“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].
說明:
儘管上面的答案是按字典序排列的，但是你可以任意選擇答案輸出的順序。

分析
由於不知道有幾個字元，所以用遞迴來做是最好理解的了，相當於套了n重的for迴圈。

c++實現

class Solution {
public:
    vector<string> res;
    
    vector<
 
string> letterCombinations(string digits) {
        if (digits == "")
            return res;
        zuhe(digits,0,"");
        return res;
    }
    void zuhe(string digits,int i,string s) {
        if (i == digits.length())
        {
            res.push_back(s);
            return;
        }
        if
 
 (digits[i] == '2')
        {
            zuhe(digits,i+1,s+"a");
            zuhe(digits,i+1,s+"b");
            zuhe(digits,i+1,s+"c");
            return;
        }
        if (digits[i] == '3')
        {
            zuhe(digits,i+1,s+"d");
            zuhe(digits,i+1,s+"e");
            zuhe(digits,i+1,s+"f");
            return;
        }
        if (digits[i] == '4')
        {
            zuhe(digits,i+1,s+"g");
            zuhe(digits,i+1,s+"h");
            zuhe(digits,i+1,s+"i");
            return;
        }
        if (digits[i] == '5')
        {
            zuhe(digits,i+1,s+"j");
            zuhe(digits,i+1,s+"k");
            zuhe(digits,i+1,s+"l");
            return;
        }
        if (digits[i] == '6')
        {
            zuhe(digits,i+1,s+"m");
            zuhe(digits,i+1,s+"n");
            zuhe(digits,i+1,s+"o");
        }
        if (digits[i] == '7')
        {
            zuhe(digits,i+1,s+"p");
            zuhe(digits,i+1,s+"q");
            zuhe(digits,i+1,s+"r");
            zuhe(digits,i+1,s+"s");
            return;
        }
        if (digits[i] == '8')
        {
            zuhe(digits,i+1,s+"t");
            zuhe(digits,i+1,s+"u");
            zuhe(digits,i+1,s+"v");
            return;
        }
        if (digits[i] == '9')
        {
            zuhe(digits,i+1,s+"w");
            zuhe(digits,i+1,s+"x");
            zuhe(digits,i+1,s+"y");
            zuhe(digits,i+1,s+"z");
            return;
        }
    }
};