17.cpp

// dfs.cpp
class Solution {
    static vector<string> letter;
    void dfs(vector<string>& letter, vector<string>& res, const string& digits,
             string cur, int i) {
        if (i == digits.size()) {
            res.push_back(cur);
            return;
        }
        for (int j = 0; j < letter[digits[i] - '0'].size(); ++j)
            dfs(letter, res, digits, cur + letter[digits[i] - '0'][j], i + 1);
    }

public:
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        if (digits.size() == 0)
            return res;
        dfs(letter, res, digits, "", 0);
        return res;
    }
};

vector<string> Solution::letter = {"",    "",    "abc",  "def", "ghi",
                                   "jkl", "mno", "pqrs", "tuv", "wxyz"};

string keymap[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
class Solution2 {
    vector<string> combine(vector<string>& a, string& b) {
        vector<string> res;
        for (auto& i : a) {
            for (auto& j : b) {
                res.push_back(i + j);
            }
        }
        return res;
    }

public:
    vector<string> letterCombinations(string digits) {
        if (digits.empty())
            return {};
        vector<string> res = {""};
        for (auto& d : digits) {
            res = combine(res, keymap[d - '2']);
        }
        return res;
    }
};