Given a digit string, 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.

Input:Digit string "23"

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

int counts[] = {, , , , , , , , , };

char letter[] = {'', '', 'a', 'd', 'g', 'j', 'm', 'p', 't', 'w'};  

class Solution {

public:

     void    DFS( string & digit,int i, string s)

     {

        if(i == size){

            result.push_back(s);

            return ;

        }

        int index = digit[i] - '' ;

        for(int j = ; j < counts[index] ; j++)

        {                        

            char c = letter[index]+j ;

            DFS(digit, i+, s+c);    

        }

    }

    vector<string> letterCombinations(string digits) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        result.clear();

        size = digits.size();

        string s = "";

        DFS(digits,  , s);

        return result ;

    }

private :

vector<string> result ;

int size ;

};