17 电话号码的字母组合

https://leetcode.cn/leetbook/read/top-interview-questions-medium/xvpj16/

cost: 20m30s

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        if (digits.size() == 0) {
            return {};
        }
        vector<string> res;
        string combination = "";
        this->walk(res, digits, 0, combination);
        return res;
    }

private:
    void walk(vector<string>& res, string& digits, int k, string& combination) {
        if (k == digits.size()) {
            res.push_back(combination);
            return;
        }
        string s = this->phoneMap[digits[k]];
        for (const char ch : s) {
            combination.push_back(ch);
            this->walk(res, digits, k + 1, combination);
            combination.pop_back();
        }
    }

    unordered_map<char, string> phoneMap = {
        {'2', "abc"}, {'3', "def"},  {'4', "ghi"}, {'5', "jkl"},
        {'6', "mno"}, {'7', "pqrs"}, {'8', "tuv"}, {'9', "wxyz"},
    };
};