/**

res，level，nums，i，dict

**/

class Solution {

public:

    vector<string> letterCombinations(string digits) {

        len=digits.length();

        if(len==) return {};

        //字母转化为数字

        for(int i=;i<len;i++){

            int num=digits[i]-'';

            nums.push_back(num);

        }

        vector<string> res;

        string level;

        dfs(,res,level);

        return res;

    }

    //建立递归函数，从第一个号码开始递归，到最后一个号码结束；

    void dfs(int i,vector<string>&res,string& level){

        //递归边界

        if(i>=len){

            res.push_back(level);return;

        }

        //递归表达式，选择第i个元素

        string cur_str=dict[nums[i]];

        for(int k=;k<cur_str.length();k++){

            level.push_back(cur_str[k]);

            dfs(i+,res,level);

            level.pop_back();

        }

    }

private:

    int len;

    vector<int> nums;

    vector<string> dict={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};

};