参考代码1

class Solution {
public:
    string path;
    vector<string> res;
    string dict[10] = {
        "", "",
        "abc", "def",
        "ghi", "jkl", "mno",
        "pqrs", "tuv", "wxyz", 
    };

    vector<string> letterCombinations(string digits) {
        if (digits.empty()) return vector<string> (); //或者return res; 也行
        dfs(digits, 0);
        return res;
    }

    void dfs(string& digits, int u) {
        if (u == digits.size()) {
            res.push_back(path);
            return ;
        }
        int ch = digits[u] - '0';
        for (int i = 0; i < dict[ch].size(); i ++) {
            string tmp = path;

            path += dict[ch][i];
            dfs(digits, u + 1);
            path = tmp; // 恢复现场
        }
}
};

yxc代码

class Solution {
public:
    vector<string> ans;
    string strs[10] = {
        "", "", "abc", "def",
        "ghi", "jkl", "mno",
        "pqrs", "tuv", "wxyz",
    };

    vector<string> letterCombinations(string digits) {
        if (digits.empty()) return ans;
        dfs(digits, 0, "");
        return ans;
    }

    void dfs(string& digits, int u, string path) {
        if (u == digits.size()) ans.push_back(path);
        else {
            for (auto c : strs[digits[u] - '0'])
                dfs(digits, u + 1, path + c);
        }
    }
};