#include <array>
#include <cassert>
#include <string>
#include <vector>

struct Solution {
    std::vector<std::string> letterCombinations(const std::string &digits) {
        solver s(digits);
        s.solve();
        return s.combinations;
    }

  private:
    struct solver {
        const std::string &digits;
        std::vector<std::string> combinations;

        solver(const std::string &digits) : digits(digits) {}
        void solve() {
            std::string s;
            solve(s, 0);
        }

      private:
        std::array<std::string, 8> LETTERS{"abc", "def",  "ghi", "jkl",
                                           "mno", "pqrs", "tuv", "wxyz"};

        void solve(std::string &s, int i) {
            if (i < 0 || i >= static_cast<int>(digits.size())) {
                if (!s.empty()) {
                    combinations.emplace_back(s);
                }

                return;
            }

            for (char c : LETTERS[digits[i] - '2']) {
                s.push_back(c);
                solve(s, i + 1);
                s.pop_back();
            }
        }
    };
};

int main() {
    Solution s;

    assert((s.letterCombinations("23") ==
            std::vector<std::string>{"ad", "ae", "af", "bd", "be", "bf", "cd",
                                     "ce", "cf"}));
    assert((s.letterCombinations("") == std::vector<std::string>{}));
    assert(
        (s.letterCombinations("2") == std::vector<std::string>{"a", "b", "c"}));

    return 0;
}