Letter Combinations of a Phone Number

https://leetcode.com/problems/letter-combinations-of-a-phone-number/

Source

    unordered_map<int, string> lut {
        {2, "abc"},
        {3, "def"},
        {4, "ghi"},
        {5, "jkl"},
        {6, "mno"},
        {7, "pqrs"},
        {8, "tuv"},
        {9, "wxyz"}
    };

    vector<string> letterCombinations(string digits) {
        hashStr hash;
        DFS(digits, 0, "", hash);
        vstr vstrAns;
        for (auto it=begin(hash); it!=end(hash); ++it) {
            vstrAns.push_back(*it);
        }
        return vstrAns;
    }

    void DFS(const string& digits, const int idx, const string& letComb, hashStr& hash) {
        if (digits.length() == 0) {
            return;
        }
        if (idx == digits.length()) {
            hash.insert(letComb);
            return;
        }

        const int num = (digits[idx] - '0');
        const string letters = lut[num];
        FOR (i, letters.length()) {
            DFS(digits, idx+1, letComb + letters.substr(i, 1), hash);
        }
    }

GitHub

LetterCombinations

LetterCombinations