LeetCode_Letter Combinations of a Phone Number

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"].

　　水题： DFS

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 ;
};