[leetcode]Letter Combinations of a Phone Number

class Solution {
public:
    string getChar(char ch){;
        switch(ch){
            case '2' : return "abc";
            case '3' : return "def";
            case '4' : return "ghi";
            case '5' : return "jkl";
            case '6' : return "mno";
            case '7' : return "pqrs";
            case '8' : return "tuv";
            case '9' : return "wxyz";
            default : return "";
        }
    }
    void search(string &digits , int index , int end , vector<string>& ans , string maybe){
        if(index >= end){
            ans.push_back(maybe);
            return ;
        }
       // cout << index << endl;
        string chars = getChar(digits[index]);
        int len = chars.size();
        for(int i = 0 ; i < len ; i++)
            search(digits , index + 1 , end , ans , maybe + chars[i]);
    }
    vector<string> letterCombinations(string digits) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<string> ans;
        if(digits == ""){
            ans.push_back("");
            return ans;
        }
        search(digits , 0 , digits.size() , ans , "");
        return ans;
    }
};