class Solution {
 public:
  vector<string> letterCombinations(string digits) {
    if (digits.empty())
      return {};

    vector<string> ans;

    dfs(digits, 0, "", ans);
    return ans;
  }

 private:
  const vector<string> digitToLetters{"",    "",    "abc",  "def", "ghi",
                                      "jkl", "mno", "pqrs", "tuv", "wxyz"};

  void dfs(const string& digits, int i, string&& path, vector<string>& ans) {
    if (i == digits.length()) {
      ans.push_back(path);
      return;
    }

    for (const char letter : digitToLetters[digits[i] - '0']) {
      path.push_back(letter);
      dfs(digits, i + 1, move(path), ans);
      path.pop_back();
    }
  }
};