leetcode--17. Letter Combinations of a Phone Number

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.

利用backtrace。当探到底时，可以push_back

class Solution {
public:
    
    unordered_map<char, string> hash;
    vector<string> ret;
    
    void bt(string digits, int index, string former_str) {
        char key = digits[index];
        string val = hash[key];
        for(int i = 0; i < val.length(); i++){
            string tmp_str = former_str + val[i];
            if(index == digits.length()-1) {
                ret.push_back(tmp_str);
            }
            else{
                bt(digits, index + 1, tmp_str);
            }
        }
    }
    
    
    vector<string> letterCombinations(string digits) {
        hash.insert({{'2',"abc"},{'3',"def"},{'4',"ghi"},{'5',"jkl"},{'6',"mno"},{'7',"pqrs"},{'8',"tuv"},{'9',"wxyz"}});  
        bt(digits, 0, "");
        return ret;
    }
};