Letter Combinations of a Phone Number

 1 class Solution
 2 {
 3 private:
 4   void Initialize()
 5   {
 6     letters.push_back("");
 7     letters.push_back("");
 8     letters.push_back("abc");
 9     letters.push_back("def");
10     letters.push_back("ghi");
11     letters.push_back("jkl");
12     letters.push_back("mno");
13     letters.push_back("pqrs");
14     letters.push_back("tuv");
15     letters.push_back("wxyz");
16   }
17 public:
18   void dfs(int dep, int maxDep, string &s, string ans)
19   {
20     if(dep == maxDep)
21     {
22       ret.push_back(ans);
23       return;
24     }
25 
26     for(int i=0; i<letters[s[dep]-‘0‘].size(); i++)
27       dfs(dep+1, maxDep, s, ans + letters[s[dep]-‘0‘][i]);
28   } 
29   vector<string> letterCombinations(string digits)
30   {
31     char ch;
32     Initialize();
33     ret.clear();
34 
35     if(digits != "")
36       dfs(0, digits.size(), digits, "");
37     
38     return ret;
39   }
40 
41 private:
42   vector<string> letters;
43   vector<string> ret;
44 };