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[] = {0, 0, 3, 3, 3, 3, 3, 4, 3, 4};  
char letter[] = {'0', '0', '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] - '0' ;
    
        for(int j = 0; j < counts[index] ; j++)
        {                        
                        
            char c = letter[index]+j ;
                      
            DFS(digit, i+1, 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, 0 , s);
        
        return result ;
        
    }
private :
vector<string> result ;
int size ;
};