电话号码的字母组合.
b站视频.

class Solution {
public:
    vector<string> res; 
    vector<string> v = {"", "" ,"abc", "def", "ghi", "jkl","mno","pqrs","tuv","wxyz"};
    vector<string> letterCombinations(string digits) {
        if(digits.empty() || digits == "") return res;
        findCombinations(digits, 0, "");
        return res;
    }
    void findCombinations(string digits, int index, string s)
    {
        if(index == digits.size()){
            res.push_back(s);
            return ;
        }
        string str = v[digits[index] - '0']; 
        for(int i = 0 ; i < str.size(); i++){
            findCombinations(digits, index + 1, s + str[i]);
        }

        return ;
    }
};