Letter Combinations of a Phone Number

class Solution {
 private:
     map<char, vector<char> > dict;
     vector<string> ret;
 public:
     void createDict()
     {
         dict.clear();
         dict['2'].push_back('a');
         dict['2'].push_back('b');
         dict['2'].push_back('c');
         dict['3'].push_back('d');
         dict['3'].push_back('e');
         dict['3'].push_back('f');
         dict['4'].push_back('g');
         dict['4'].push_back('h');
         dict['4'].push_back('i');
         dict['5'].push_back('j');
         dict['5'].push_back('k');
         dict['5'].push_back('l');
         dict['6'].push_back('m');
         dict['6'].push_back('n');
         dict['6'].push_back('o');
         dict['7'].push_back('p');
         dict['7'].push_back('q');
         dict['7'].push_back('r');
         dict['7'].push_back('s');
         dict['8'].push_back('t');
         dict['8'].push_back('u');
         dict['8'].push_back('v');
         dict['9'].push_back('w');
         dict['9'].push_back('x');
         dict['9'].push_back('y');
         dict['9'].push_back('z');
     }
     
     void dfs(int dep, int maxDep, string s, string ans)
     {
         if (dep == maxDep)
         {
             ret.push_back(ans);
             return;
         }
         
         for(int i = 0; i < dict[s[dep]].size(); i++)
             dfs(dep + 1, maxDep, s, ans + dict[s[dep]][i]);
     }
     
     vector<string> letterCombinations(string digits) {
        
         ret.clear();
         createDict();
         dfs(0, digits.size(), digits, "");
         return ret;
     }
 };