[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.

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 {
    unordered_map<char, vector<string>> ht  = {
             {'0', {" "}}, {'1', {""}}, {'2', {"a", "b", "c"}}, {'3', {"d", "e", "f"}},
             {'4', {"g", "h", "i"}}, {'5', {"j", "k", "l"}}, {'6', {"m", "n", "o"}},
             {'7', {"p", "q", "r", "s"}}, {'8', {"t", "u", "v"}}, {'9', {"w", "x", "y", "z"}},
    };       
public:
    vector<string> letterCombinations(string digits) {
        vector<string> ret;
        int n = digits.size();
        
        if (n == 0) return ret;
    
        if (n == 1) return ht[digits[0]];
       
        vector<string> prev = letterCombinations(digits.substr(0, n - 1));

        for (auto s : prev){
            for (int i = 0; i < ht[digits[n-1]].size(); i++)
            ret.push_back(s+ht[digits[n-1]][i]);
        }
        
        return ret;
    }
};