【LeetCode热题100】【回溯】电话号码的字母组合

class Solution {
public:
    string code[8] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    vector<string> ans;
    string digits;

    void dfs(string combine, int depth) {
        if (depth == digits.size())
            ans.push_back(std::move(combine));
        else
            for (auto &it: code[digits[depth] - '2'])
                dfs(combine + it, depth + 1);
    }

    vector<string> letterCombinations(string digits) {
        this->digits = digits;
        if (digits.size() == 0)return {};
        dfs("", 0);
        return ans;
    }
};