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"]. 

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

class Solution {
public:
    vector< string> letterCombinations( string digits) 
    {
         string code[]={ "", "", " abc ", " def ", " ghi ", " jkl ", " mno ", " pqrs ", " tuv ", " wxyz "};
        vector< string> result;
         if(digits.length()== 0)
        {
            result.push_back( "");
             return result;
        }
        
         string v; for( int i= 0;i<digits.length();i++) v=v+ "   ";
        generate(code,result,v,digits, 0);
         return result;
    }
    
     void generate( string* code,vector< string>& result, string& v, string& digits, int dep)
    {
         if(dep==digits.length())
        {
            result.push_back(v);
             return;
        }
         for( int i= 0;i<code[digits[dep]- ' 0 '].length();i++)
        {
            v[dep]=code[digits[dep]- ' 0 '][i];
            generate(code,result,v,digits,dep+ 1);
        }
    }
};