casc

 string getKthCombination(string digits, int k) {
        if (digits.empty()) return "";

        numberMap[2] = "abc";
        numberMap[3] = "def";
        numberMap[4] = "ghi";
        numberMap[5] = "jkl";
        numberMap[6] = "mno";
        numberMap[7] = "pqrs";
        numberMap[8] = "tuv";
        numberMap[9] = "wxyz";

        int n = digits.size();
        vector<int> sizes(n, 1);

        for (int i = n - 2; i >= 0; --i) {
            sizes[i] = sizes[i + 1] * numberMap[digits[i + 1] - '0'].size();
        }

        string result;
        --k;

        for (int i = 0; i < n; ++i) {
            int digit = digits[i] - '0';
            string possibleChars = numberMap[digit];
            int idx = k / sizes[i];
            result += possibleChars[idx];
            k %= sizes[i];
        }

        return result;
    }