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
     
       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
      
        result ;int size ;};