const char* lookup[] = {
   " ", "", "abc", "def",                                "ghi", "jkl", "mno",                                 "pqrs", "tuv", "wxyz"                         };class Solution {public:    vector
     
       letterCombinations(string digits) {           string str;        vector
      
        res;        dfs(digits, 0, str, res);        return res;    }        void dfs(string& digits, int pos, string& str, vector
       
        & res) {        if (digits.length() <=  pos) {            res.push_back(str);            return;        }                const char* alphabetas = lookup[digits[pos] - '0'];        for (int i=0; alphabetas[i] != '\0'; i++) {            str.push_back(alphabetas[i]);            dfs(digits, pos + 1, str, res);            str.pop_back();        }    }};
       
      
     

dfs