Monogram] [LeetCode phone number

[Problems] Given a string that contains only numbers 2-9, it can return all letter combinations indicated.

Given digital map to letters as follows (the same telephone key). Note 1 does not correspond to any alphabet.

 

 Example:

Input: " 23 " 
Output: [ " AD " , " AE " , " AF " , " BD " , " BE " , " BF " , " CD " , " CE " , " CF2 " .] 
Description: 
Although the above the answer is based on lexicographic order, but you can choose the order of the answer output.

[Thinking] This is a retrospective issue of the Normal template, we first think back exit condition: the size and the size of the array digits us search out tmp_res phase exit at the same time, because we all share the function of each back tmp_res this variable, so after completion of the function call needs to be restored to its original state , namely:
tmp_res = I +
backtrace (......)
tmp_res.pop_back ()
wherein there are two parameters in the traceback function is varied, a character string is searched out, the other It is the digits of the index, because it is based on the numerical digits to select the add those letters! ! !

class Solution {
public:
    void backtrace(vector<string>& res, string tmp_res, string digits, int idx){  // res为引用形式
        if(tmp_res.length() == digits.length()){
            res.push_back(tmp_res);
            return;
        }
        string str = table[digits[idx]];
        for(auto i: str){
            tmp_res += i;
            backtrace(res, tmp_res, digits, idx+1);
            tmp_res.pop_back();
        }
    }

    vector<string> letterCombinations(string digits) {

        vector<string> res;
        if(digits == "")  return res;
        backtrace(res, "", digits, 0);
        return res;
    }
private:
    unordered_map<char, string> table{{'0', " "}, {'1',"*"}, {'2', "abc"},
            {'3',"def"}, {'4',"ghi"}, {'5',"jkl"},
            {'6',"mno"}, {'7',"pqrs"},{'8',"tuv"},
            {'9',"wxyz"}};
};