Letter Combinations of a Phone Number——简单的回溯算法

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 {
private:
    vector<string> res;
    string temp;
public:
    void getRes(string digits,int len,map<char,string> num_map,int index)
    {
        if(index>=len)
        {
            res.push_back(temp);
            return;
        }
        char   strNum=digits[index];
        string strLetter=num_map[strNum];
        for(int j=;j<strLetter.size();j++)
        {
            temp.push_back(strLetter[j]);
            getRes(digits,len,num_map,index+);
            temp.pop_back();
        }

    }
    vector<string> letterCombinations(string digits) {
        map<char,string> num_map;
        num_map['']="";
        num_map['']="";
        num_map['']="abc";
        num_map['']="def";
        num_map['']="ghi";
        num_map['']="jkl";
        num_map['']="mno";
        num_map['']="pqrs";
        num_map['']="tuv";
        num_map['']="wxyz";
        int len=digits.size();
        if(len==)
            return res;
        getRes(digits,len,num_map,);
        return res;
    }
};